diff --git a/core/lib/Drupal/Core/Workflow/Entity/Workflow.php b/core/lib/Drupal/Core/Workflow/Entity/Workflow.php
index b84af09..b57751d 100644
--- a/core/lib/Drupal/Core/Workflow/Entity/Workflow.php
+++ b/core/lib/Drupal/Core/Workflow/Entity/Workflow.php
@@ -245,10 +245,6 @@ public function addTransition($transition_id, $label, array $from_state_ids, $to
     if (preg_match('/[^a-z0-9_]+/', $transition_id)) {
       throw new \InvalidArgumentException("The transition ID '$transition_id' must contain only lowercase letters, numbers, and underscores");
     }
-    // @todo what to about duplicates - I don't think this needs to be in the
-    //   API.
-    // if (!$this->hasTransitionFromStateToState($from_state_id, $to_state_id)) {
-    // }
 
     if (!$this->hasState($to_state_id)) {
       throw new \InvalidArgumentException("The state '$to_state_id' does not exist in workflow '{$this->id()}'");
@@ -258,7 +254,13 @@ public function addTransition($transition_id, $label, array $from_state_ids, $to
       'label' => $label,
     ];
 
-    $this->setTransitionFromStates($transition_id, $from_state_ids);
+    try {
+      $this->setTransitionFromStates($transition_id, $from_state_ids);
+    }
+    catch (\InvalidArgumentException $e) {
+      unset($this->transitions[$transition_id]);
+      throw $e;
+    }
 
     return $this;
   }
@@ -330,36 +332,39 @@ public function getTransitionsForState($state_id, $direction = 'from') {
   /**
    * {@inheritdoc}
    */
-  public function getTransitionsFromStateToState($from_state_id, $to_state_id) {
-    $transition_ids = $this->getTransitionIdsFromStateToState($from_state_id, $to_state_id);
-    if (empty($transition_ids)) {
+  public function getTransitionFromStateToState($from_state_id, $to_state_id) {
+    $transition_id = $this->getTransitionIdFromStateToState($from_state_id, $to_state_id);
+    if (empty($transition_id)) {
       throw new \InvalidArgumentException("The transition from '$from_state_id' to '$to_state_id' does not exist in workflow '{$this->id()}'");
     }
-    return $this->getTransitions($transition_ids);
+    return $this->getTransition($transition_id);
   }
 
   /**
    * {@inheritdoc}
    */
   public function hasTransitionFromStateToState($from_state_id, $to_state_id) {
-    return !empty($this->getTransitionIdsFromStateToState($from_state_id, $to_state_id));
+    return !empty($this->getTransitionIdFromStateToState($from_state_id, $to_state_id));
   }
 
   /**
-   * Gets the possible transition IDs from state to state.
+   * Gets the transition ID from state to state.
    *
    * @param string $from_state_id
    *   The state ID to transition from.
    * @param string $to_state_id
    *   The state ID to transition to.
    *
-   * @return string[]
-   *   The transition IDs.
+   * @return string|null
+   *   The transition ID, or NULL if no transition exists.
    */
-  protected function getTransitionIdsFromStateToState($from_state_id, $to_state_id) {
-    return array_keys(array_filter($this->transitions, function ($transition) use ($from_state_id, $to_state_id) {
-      return in_array($from_state_id, $transition['from'], TRUE) && $transition['to'] === $to_state_id;
-    }));
+  protected function getTransitionIdFromStateToState($from_state_id, $to_state_id) {
+    foreach ($this->transitions as $transition_id => $transition) {
+      if (!empty($transition['from']) && in_array($from_state_id, $transition['from'], TRUE) && $transition['to'] === $to_state_id) {
+        return $transition_id;
+      }
+    }
+    return NULL;
   }
 
   /**
@@ -388,6 +393,12 @@ public function setTransitionFromStates($transition_id, array $from_state_ids) {
       if (!$this->hasState($from_state_id)) {
         throw new \InvalidArgumentException("The state '$from_state_id' does not exist in workflow '{$this->id()}'");
       }
+      if ($this->hasTransitionFromStateToState($from_state_id, $this->transitions[$transition_id]['to'])) {
+        $transition = $this->getTransitionFromStateToState($from_state_id, $this->transitions[$transition_id]['to']);
+        if ($transition_id !== $transition->id()) {
+          throw new \InvalidArgumentException("The '{$transition->id()}' transition already allows '$from_state_id' to '{$this->transitions[$transition_id]['to']}' transitions in workflow '{$this->id()}'");
+        }
+      }
     }
 
     // Preserve the order of the state IDs in the from value and don't save any
diff --git a/core/lib/Drupal/Core/Workflow/State.php b/core/lib/Drupal/Core/Workflow/State.php
index 3a8b6ae..386e52e 100644
--- a/core/lib/Drupal/Core/Workflow/State.php
+++ b/core/lib/Drupal/Core/Workflow/State.php
@@ -89,11 +89,11 @@ public function canTransitionTo($to_state_id) {
   /**
    * {@inheritdoc}
    */
-  public function getTransitionsTo($to_state_id) {
+  public function getTransitionTo($to_state_id) {
     if (!$this->canTransitionTo($to_state_id)) {
       throw new \InvalidArgumentException("Can not transition to '$to_state_id' state");
     }
-    return $this->workflow->getTransitionsFromStateToState($this->id(), $to_state_id);
+    return $this->workflow->getTransitionFromStateToState($this->id(), $to_state_id);
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Workflow/StateInterface.php b/core/lib/Drupal/Core/Workflow/StateInterface.php
index 35463f4..524f94b 100644
--- a/core/lib/Drupal/Core/Workflow/StateInterface.php
+++ b/core/lib/Drupal/Core/Workflow/StateInterface.php
@@ -49,18 +49,18 @@ public function weight();
   public function canTransitionTo($to_state_id);
 
   /**
-   * Gets the Transition objects for the provided state ID.
+   * Gets the Transition object for the provided state ID.
    *
    * @param $to_state_id
    *   The state to transition to.
    *
-   * @return \Drupal\Core\Workflow\TransitionInterface[]
-   *   The Transition objects for the provided state ID.
+   * @return \Drupal\Core\Workflow\TransitionInterface
+   *   The Transition object for the provided state ID.
    *
    * @throws \InvalidArgumentException()
    *   Exception thrown when the provided state ID can not be transitioned to.
    */
-  public function getTransitionsTo($to_state_id);
+  public function getTransitionTo($to_state_id);
 
   /**
    * Gets all the possible transition objects for the state.
diff --git a/core/lib/Drupal/Core/Workflow/WorkflowInterface.php b/core/lib/Drupal/Core/Workflow/WorkflowInterface.php
index 9ec731a..e42baf5 100644
--- a/core/lib/Drupal/Core/Workflow/WorkflowInterface.php
+++ b/core/lib/Drupal/Core/Workflow/WorkflowInterface.php
@@ -195,13 +195,13 @@ public function getTransitionsForState($state_id, $direction = 'from');
    * @param string $to_state_id
    *   The state ID to transition to.
    *
-   * @return \Drupal\Core\Workflow\TransitionInterface[]
-   *   The possible transitions.
+   * @return \Drupal\Core\Workflow\TransitionInterface
+   *   The transitions.
    *
    * @throws \InvalidArgumentException
    *   Thrown if the transition does not exist.
    */
-  public function getTransitionsFromStateToState($from_state_id, $to_state_id);
+  public function getTransitionFromStateToState($from_state_id, $to_state_id);
 
   /**
    * Determines if a transition from state to state exists.
diff --git a/core/modules/content_moderation/src/ContentModerationState.php b/core/modules/content_moderation/src/ContentModerationState.php
index 6a1d686..265a01c 100644
--- a/core/modules/content_moderation/src/ContentModerationState.php
+++ b/core/modules/content_moderation/src/ContentModerationState.php
@@ -100,8 +100,8 @@ public function canTransitionTo($to_state_id) {
   /**
    * {@inheritdoc}
    */
-  public function getTransitionsTo($to_state_id) {
-    return $this->state->getTransitionsTo($to_state_id);
+  public function getTransitionTo($to_state_id) {
+    return $this->state->getTransitionTo($to_state_id);
   }
 
   /**
diff --git a/core/modules/system/tests/modules/workflow_type_test/src/DecoratedState.php b/core/modules/system/tests/modules/workflow_type_test/src/DecoratedState.php
index 0e6b567..dfd1169 100644
--- a/core/modules/system/tests/modules/workflow_type_test/src/DecoratedState.php
+++ b/core/modules/system/tests/modules/workflow_type_test/src/DecoratedState.php
@@ -76,8 +76,8 @@ public function canTransitionTo($to_state_id) {
   /**
    * {@inheritdoc}
    */
-  public function getTransitionsTo($to_state_id) {
-    return $this->state->getTransitionsTo($to_state_id);
+  public function getTransitionTo($to_state_id) {
+    return $this->state->getTransitionTo($to_state_id);
   }
 
   /**
diff --git a/core/modules/workflow_ui/src/Form/WorkflowTransitionAddForm.php b/core/modules/workflow_ui/src/Form/WorkflowTransitionAddForm.php
index be495f9..a33608c 100644
--- a/core/modules/workflow_ui/src/Form/WorkflowTransitionAddForm.php
+++ b/core/modules/workflow_ui/src/Form/WorkflowTransitionAddForm.php
@@ -110,16 +110,16 @@ protected function copyFormValuesToEntity(EntityInterface $entity, array $form,
    */
   public function validateForm(array &$form, FormStateInterface $form_state) {
     /** @var \Drupal\Core\Workflow\WorkflowInterface $workflow */
-    // @todo should we only permit single transitions through the UI?
-    //$workflow = $this->getEntity();
-    //$values = $form_state->getValues();
-    // If the form is for a new transition it should not exist.
-    // if ($workflow->hasTransitionFromStateToState($values['from'], $values['to'])) {
-    //  $form_state->setErrorByName('from', $this->t('The transition from %from to %to already exists.', [
-    //    '%from' => $workflow->getState($values['from'])->label(),
-    //    '%to' => $workflow->getState($values['to'])->label(),
-    //  ]));
-    //}
+    $workflow = $this->getEntity();
+    $values = $form_state->getValues();
+    foreach (array_filter($values['from']) as $from_state_id) {
+      if ($workflow->hasTransitionFromStateToState($from_state_id, $values['to'])) {
+        $form_state->setErrorByName('from][' . $from_state_id, $this->t('The transition from %from to %to already exists.', [
+          '%from' => $workflow->getState($from_state_id)->label(),
+          '%to' => $workflow->getState($values['to'])->label(),
+        ]));
+      }
+    }
   }
 
   /**
diff --git a/core/modules/workflow_ui/src/Form/WorkflowTransitionDeleteForm.php b/core/modules/workflow_ui/src/Form/WorkflowTransitionDeleteForm.php
index b007fa3..f49f41f 100644
--- a/core/modules/workflow_ui/src/Form/WorkflowTransitionDeleteForm.php
+++ b/core/modules/workflow_ui/src/Form/WorkflowTransitionDeleteForm.php
@@ -44,7 +44,7 @@ public function getFormId() {
    * {@inheritdoc}
    */
   public function getQuestion() {
-    return $this->t('Are you sure you want to delete the %transition transition from %workflow?', ['%transition' => $this->transition->label(), '%workflow' => $this->workflow->label()]);
+    return $this->t('Are you sure you want to delete %transition from %workflow?', ['%transition' => $this->transition->label(), '%workflow' => $this->workflow->label()]);
   }
 
   /**
diff --git a/core/modules/workflow_ui/src/Form/WorkflowTransitionEditForm.php b/core/modules/workflow_ui/src/Form/WorkflowTransitionEditForm.php
index ed92497..313bda7 100644
--- a/core/modules/workflow_ui/src/Form/WorkflowTransitionEditForm.php
+++ b/core/modules/workflow_ui/src/Form/WorkflowTransitionEditForm.php
@@ -79,6 +79,26 @@ public function form(array $form, FormStateInterface $form_state) {
   }
 
   /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    /** @var \Drupal\Core\Workflow\WorkflowInterface $workflow */
+    $workflow = $this->getEntity();
+    $values = $form_state->getValues();
+    foreach (array_filter($values['from']) as $from_state_id) {
+      if ($workflow->hasTransitionFromStateToState($from_state_id, $values['to'])) {
+        $transition = $workflow->getTransitionFromStateToState($from_state_id, $values['to']);
+        if ($transition->id() !== $values['id']) {
+          $form_state->setErrorByName('from][' . $from_state_id, $this->t('The transition from %from to %to already exists.', [
+            '%from' => $workflow->getState($from_state_id)->label(),
+            '%to' => $workflow->getState($values['to'])->label(),
+          ]));
+        }
+      }
+    }
+  }
+
+  /**
    * Copies top-level form values to entity properties
    *
    * This form can only change values for a state, which is part of workflow.
diff --git a/core/modules/workflow_ui/tests/src/Functional/WorkflowUiTest.php b/core/modules/workflow_ui/tests/src/Functional/WorkflowUiTest.php
index 2db9b64..5f3c1c4 100644
--- a/core/modules/workflow_ui/tests/src/Functional/WorkflowUiTest.php
+++ b/core/modules/workflow_ui/tests/src/Functional/WorkflowUiTest.php
@@ -123,11 +123,30 @@ public function testWorkflowCreation() {
     $this->clickLink('Add a new transition');
     $this->submitForm(['id' => 'create_new_draft', 'label' => 'Create new draft', 'from[published]' => 'published', 'to' => 'draft'], 'Save');
     $this->assertSession()->pageTextContains('The machine-readable name is already in use. It must be unique.');
+    // Try creating a transition which duplicates the states of another.
+    $this->submitForm(['id' => 'create_new_draft2', 'label' => 'Create new draft again', 'from[published]' => 'published', 'to' => 'draft'], 'Save');
+    $this->assertSession()->pageTextContains('The transition from Live to Draft already exists.');
+
+    // Create a new transition.
+    $this->submitForm(['id' => 'save_and_publish', 'label' => 'Save and publish', 'from[published]' => 'published', 'to' => 'published'], 'Save');
+    $this->assertSession()->pageTextContains('Created Save and publish transition.');
+    // Edit the new transition and try to add an existing transition.
+    $this->clickLink('Edit', 3);
+    $this->submitForm(['from[draft]' => 'draft'], 'Save');
+    $this->assertSession()->pageTextContains('The transition from Draft to Live already exists.');
+
+    // Delete the transition.
+    $workflow = $workflow_storage->loadUnchanged('test');
+    $this->assertTrue($workflow->hasTransitionFromStateToState('published', 'published'), 'Can transition from published to published');
+    $this->clickLink('Delete');
+    $this->assertSession()->pageTextContains('Are you sure you want to delete Save and publish from Test?');
+    $this->submitForm([], 'Delete');
+    $workflow = $workflow_storage->loadUnchanged('test');
+    $this->assertFalse($workflow->hasTransitionFromStateToState('published', 'published'), 'Cannot transition from published to published');
 
     // Try creating a duplicate state.
     $this->drupalGet('admin/config/workflow/workflows/manage/test');
     $this->clickLink('Add a new state');
-    // Don't create a draft to draft transition by default.
     $this->submitForm(['label' => 'Draft', 'id' => 'draft'], 'Save');
     $this->assertSession()->pageTextContains('The machine-readable name is already in use. It must be unique.');
 
diff --git a/core/tests/Drupal/Tests/Core/Workflow/StateTest.php b/core/tests/Drupal/Tests/Core/Workflow/StateTest.php
index 218f809..f2509ed 100644
--- a/core/tests/Drupal/Tests/Core/Workflow/StateTest.php
+++ b/core/tests/Drupal/Tests/Core/Workflow/StateTest.php
@@ -73,28 +73,28 @@ public function testCanTransitionTo() {
   }
 
   /**
-   * @covers ::getTransitionsTo
+   * @covers ::getTransitionTo
    */
-  public function testGetTransitionsTo() {
+  public function testGetTransitionTo() {
     $workflow = new Workflow(['id' => 'test', 'type' => 'test_type'], 'workflow');
     $workflow
       ->addState('draft', 'Draft')
       ->addState('published', 'Published')
       ->addTransition('publish', 'Publish', ['draft'], 'published');
     $state = $workflow->getState('draft');
-    $transition = $state->getTransitionsTo('published');
-    $this->assertEquals('Publish', $transition['publish']->label());
+    $transition = $state->getTransitionTo('published');
+    $this->assertEquals('Publish', $transition->label());
   }
 
   /**
-   * @covers ::getTransitionsTo
+   * @covers ::getTransitionTo
    */
-  public function testGetTransitionsToException() {
+  public function testGetTransitionToException() {
     $this->setExpectedException(\InvalidArgumentException::class, "Can not transition to 'published' state");
     $workflow = new Workflow(['id' => 'test', 'type' => 'test_type'], 'workflow');
     $workflow->addState('draft', 'Draft');
     $state = $workflow->getState('draft');
-    $state->getTransitionsTo('published');
+    $state->getTransitionTo('published');
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Core/Workflow/TransitionTest.php b/core/tests/Drupal/Tests/Core/Workflow/TransitionTest.php
index 8225d55..b55ce0f 100644
--- a/core/tests/Drupal/Tests/Core/Workflow/TransitionTest.php
+++ b/core/tests/Drupal/Tests/Core/Workflow/TransitionTest.php
@@ -63,7 +63,7 @@ public function testFromAndTo() {
       ->addState('published', 'Published')
       ->addTransition('publish', 'Publish', ['draft'], 'published');
     $state = $workflow->getState('draft');
-    $transition = $state->getTransitionsTo('published')['publish'];
+    $transition = $state->getTransitionTo('published');
     $this->assertEquals($state, $transition->from()['draft']);
     $this->assertEquals($workflow->getState('published'), $transition->to());
   }
diff --git a/core/tests/Drupal/Tests/Core/Workflow/WorkflowTest.php b/core/tests/Drupal/Tests/Core/Workflow/WorkflowTest.php
index dc1f9b0..aeb592d 100644
--- a/core/tests/Drupal/Tests/Core/Workflow/WorkflowTest.php
+++ b/core/tests/Drupal/Tests/Core/Workflow/WorkflowTest.php
@@ -144,7 +144,7 @@ public function testGetState() {
     $this->assertTrue($draft->canTransitionTo('draft'));
     $this->assertTrue($draft->canTransitionTo('published'));
     $this->assertFalse($draft->canTransitionTo('archived'));
-    $this->assertEquals('Publish', $draft->getTransitionsTo('published')['publish']->label());
+    $this->assertEquals('Publish', $draft->getTransitionTo('published')->label());
     $this->assertEquals(0, $draft->weight());
     $this->assertEquals(1, $workflow->getState('published')->weight());
     $this->assertEquals(2, $workflow->getState('archived')->weight());
@@ -313,6 +313,36 @@ public function testAddTransitionMissingFromException() {
   /**
    * @covers ::addTransition
    */
+  public function testAddTransitionDuplicateTransitionStatesException() {
+    $this->setExpectedException(\InvalidArgumentException::class, "The 'publish' transition already allows 'draft' to 'published' transitions in workflow 'test'");
+    $workflow = new Workflow(['id' => 'test', 'type' => 'test_type'], 'workflow');
+    $workflow
+      ->addState('draft', 'Draft')
+      ->addState('published', 'Published');
+    $workflow->addTransition('publish', 'Publish', ['draft', 'published'], 'published');
+    $workflow->addTransition('draft_to_published', 'Publish a draft', ['draft'], 'published');
+  }
+
+  /**
+   * @covers ::addTransition
+   */
+  public function testAddTransitionConsistentAfterFromCatch() {
+    $workflow = new Workflow(['id' => 'test', 'type' => 'test_type'], 'workflow');
+    $workflow->addState('published', 'Published');
+    try {
+      $workflow->addTransition('publish', 'Publish', ['draft'], 'published');
+    }
+    catch (\InvalidArgumentException $e) {
+    }
+    // Ensure that the workflow is not left in an inconsistent state after an
+    // exception is thrown from Workflow::setTransitionFromStates() whilst
+    // calling Workflow::addTransition().
+    $this->assertFalse($workflow->hasTransition('publish'));
+  }
+
+  /**
+   * @covers ::addTransition
+   */
   public function testAddTransitionMissingToException() {
     $this->setExpectedException(\InvalidArgumentException::class, "The state 'published' does not exist in workflow 'test'");
     $workflow = new Workflow(['id' => 'test', 'type' => 'test_type'], 'workflow');
@@ -417,10 +447,10 @@ public function testGetTransitionsForState() {
 
 
   /**
-   * @covers ::getTransitionsFromStateToState
+   * @covers ::getTransitionFromStateToState
    * @covers ::hasTransitionFromStateToState
    */
-  public function testGetTransitionsFromStateToState() {
+  public function testGetTransitionFromStateToState() {
     $workflow = new Workflow(['id' => 'test', 'type' => 'test_type'], 'workflow');
     // By default states are ordered in the order added.
     $workflow
@@ -433,14 +463,14 @@ public function testGetTransitionsFromStateToState() {
 
     $this->assertTrue($workflow->hasTransitionFromStateToState('draft', 'published'));
     $this->assertFalse($workflow->hasTransitionFromStateToState('archived', 'archived'));
-    $transitions = $workflow->getTransitionsFromStateToState('published', 'archived');
-    $this->assertEquals('Archive', $transitions['archive']->label());
+    $transition = $workflow->getTransitionFromStateToState('published', 'archived');
+    $this->assertEquals('Archive', $transition->label());
   }
 
   /**
-   * @covers ::getTransitionsFromStateToState
+   * @covers ::getTransitionFromStateToState
    */
-  public function testGetTransitionsFromStateToStateException() {
+  public function testGetTransitionFromStateToStateException() {
     $this->setExpectedException(\InvalidArgumentException::class, "The transition from 'archived' to 'archived' does not exist in workflow 'test'");
     $workflow = new Workflow(['id' => 'test', 'type' => 'test_type'], 'workflow');
     // By default states are ordered in the order added.
@@ -452,7 +482,7 @@ public function testGetTransitionsFromStateToStateException() {
       ->addTransition('publish', 'Publish', ['draft', 'published'], 'published')
       ->addTransition('archive', 'Archive', ['published'], 'archived');
 
-    $workflow->getTransitionsFromStateToState('archived', 'archived');
+    $workflow->getTransitionFromStateToState('archived', 'archived');
   }
 
   /**