diff --git a/src/Form/DelivererConfigurationForm.php b/src/Form/DelivererConfigurationForm.php
index d291dbd..a6eb0ca 100644
--- a/src/Form/DelivererConfigurationForm.php
+++ b/src/Form/DelivererConfigurationForm.php
@@ -38,6 +38,15 @@ class DelivererConfigurationForm extends PluginConfigurationForm {
   /**
    * {@inheritdoc}
    */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    if ($this->exists($form_state->getValue('id'))) {
+      $form_state->setErrorByName('id', $this->t('A plugin with @id already exists.', ['@id' => $form_state->getValue('id')]));
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function buildForm(array $form, FormStateInterface $form_state) {
     $form = parent::buildForm($form, $form_state);
     $entity = $this->getEntity();
diff --git a/src/Tests/InmailWebTest.php b/src/Tests/InmailWebTest.php
index 963803e..cafe9a4 100644
--- a/src/Tests/InmailWebTest.php
+++ b/src/Tests/InmailWebTest.php
@@ -108,6 +108,19 @@ class InmailWebTest extends WebTestBase {
     $this->assertUrl('admin/config/system/inmail/deliverers');
     $this->assertText('Test IMAP Fetcher');
 
+    // Add an IMAP fetcher with an existing id.
+    $this->drupalGet('admin/config/system/inmail/deliverers/add');
+    // Select the IMAP plugin.
+    $edit = array(
+      'label' => 'Test IMAP Fetcher',
+      'id' => 'test_imap',
+      'plugin' => 'imap',
+    );
+    $this->drupalPostAjaxForm(NULL, $edit, 'plugin');
+    // Try to save and assert message for existing plugin id.
+    $this->drupalPostForm(NULL, $edit, 'Save');
+    $this->assertText('A plugin with test_imap already exists.');
+
     // Add a Drush deliverer. It implements different interfaces and
     // PluginConfigurationForm has to support that.
     $this->drupalGet('admin/config/system/inmail/deliverers/add');