diff --git a/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
index 7a4a4b0..dd6481e 100644
--- a/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
+++ b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
@@ -278,6 +278,7 @@ public function preSave(EntityStorageInterface $storage) {
       throw new ConfigDuplicateUUIDException(String::format('Attempt to save a configuration entity %id with UUID %uuid when this UUID is already used for %matched', array('%id' => $this->id(), '%uuid' => $this->uuid(), '%matched' => $matched_entity)));
     }
 
+    // If this entity is not new, load the original entity for comparison.
     if (!$this->isNew()) {
       $original = $storage->loadUnchanged($this->getOriginalId());
       // Ensure that the UUID cannot be changed for an existing entity.
diff --git a/core/lib/Drupal/Core/Config/Entity/ConfigEntityStorage.php b/core/lib/Drupal/Core/Config/Entity/ConfigEntityStorage.php
index 9a3bf63..2703632 100644
--- a/core/lib/Drupal/Core/Config/Entity/ConfigEntityStorage.php
+++ b/core/lib/Drupal/Core/Config/Entity/ConfigEntityStorage.php
@@ -357,7 +357,9 @@ public function save(EntityInterface $entity) {
       $this->invokeHook('insert', $entity);
     }
 
-    // Immediately update the original ID.
+    // After saving, this is now the "original entity", and subsequent saves
+    // will be updates instead of inserts, and updates must always be able to
+    // correctly identify the original entity.
     $entity->setOriginalId($entity->id());
 
     unset($entity->original);
diff --git a/core/lib/Drupal/Core/Entity/Entity.php b/core/lib/Drupal/Core/Entity/Entity.php
index ccd4afa..5e43af3 100644
--- a/core/lib/Drupal/Core/Entity/Entity.php
+++ b/core/lib/Drupal/Core/Entity/Entity.php
@@ -313,6 +313,7 @@ public function delete() {
   public function createDuplicate() {
     $duplicate = clone $this;
     $entity_type = $this->getEntityType();
+    // Reset the entity ID and indicate that this is a new entity.
     $duplicate->{$entity_type->getKey('id')} = NULL;
     $duplicate->enforceIsNew();
 
diff --git a/core/modules/breakpoint/lib/Drupal/breakpoint/Entity/Breakpoint.php b/core/modules/breakpoint/lib/Drupal/breakpoint/Entity/Breakpoint.php
index 3f158db..5fa0260 100644
--- a/core/modules/breakpoint/lib/Drupal/breakpoint/Entity/Breakpoint.php
+++ b/core/modules/breakpoint/lib/Drupal/breakpoint/Entity/Breakpoint.php
@@ -109,6 +109,8 @@ class Breakpoint extends ConfigEntityBase implements BreakpointInterface {
    * {@inheritdoc}
    */
   public function id() {
+    // If no ID is specified, build one from the properties that uniquely define
+    // this breakpoint.
     if (!isset($this->id)) {
       $this->id = $this->sourceType . '.' . $this->source . '.' . $this->name;
     }
diff --git a/core/modules/breakpoint/lib/Drupal/breakpoint/Entity/BreakpointGroup.php b/core/modules/breakpoint/lib/Drupal/breakpoint/Entity/BreakpointGroup.php
index 2f137b2..595ca76 100644
--- a/core/modules/breakpoint/lib/Drupal/breakpoint/Entity/BreakpointGroup.php
+++ b/core/modules/breakpoint/lib/Drupal/breakpoint/Entity/BreakpointGroup.php
@@ -111,6 +111,8 @@ public function save() {
    * {@inheritdoc}
    */
   public function id() {
+    // If no ID is specified, build one from the properties that uniquely define
+    // this breakpoint group.
     if (!isset($this->id)) {
       $this->id = $this->sourceType . '.' . $this->source . '.' . $this->name;
     }