diff --git a/UPGRADE.md b/UPGRADE.md
index b7d099fe9b..326d17de07 100644
--- a/UPGRADE.md
+++ b/UPGRADE.md
@@ -1,5 +1,19 @@
 # Upgrade to 2.16
 
+## Deprecated accepting duplicate IDs in the identity map
+
+For any given entity class and ID value, there should be only one object instance
+representing the entity. 
+
+In https://github.com/doctrine/orm/pull/10785, a check was added that will guard this
+in the identity map. The most probable cause for violations of this rule are collisions
+of application-provided IDs.
+
+In ORM 2.16.0, the check was added by throwing an exception. In ORM 2.16.1, this will be
+changed to a deprecation notice. ORM 3.0 will make it an exception again. Use
+`\Doctrine\ORM\Configuration::setRejectIdCollisionInIdentityMap()` if you want to opt-in
+to the new mode.
+
 ## Potential changes to the order in which `INSERT`s are executed
 
 In https://github.com/doctrine/orm/pull/10547, the commit order computation was improved
diff --git a/lib/Doctrine/ORM/Configuration.php b/lib/Doctrine/ORM/Configuration.php
index fcde0c190f..ca5063667c 100644
--- a/lib/Doctrine/ORM/Configuration.php
+++ b/lib/Doctrine/ORM/Configuration.php
@@ -1117,4 +1117,14 @@ public function setLazyGhostObjectEnabled(bool $flag): void
 
         $this->_attributes['isLazyGhostObjectEnabled'] = $flag;
     }
+
+    public function setRejectIdCollisionInIdentityMap(bool $flag): void
+    {
+        $this->_attributes['rejectIdCollisionInIdentityMap'] = $flag;
+    }
+
+    public function isRejectIdCollisionInIdentityMapEnabled(): bool
+    {
+        return $this->_attributes['rejectIdCollisionInIdentityMap'] ?? false;
+    }
 }
diff --git a/lib/Doctrine/ORM/UnitOfWork.php b/lib/Doctrine/ORM/UnitOfWork.php
index f9b8cf2b89..64ca810e78 100644
--- a/lib/Doctrine/ORM/UnitOfWork.php
+++ b/lib/Doctrine/ORM/UnitOfWork.php
@@ -1635,8 +1635,10 @@ public function addToIdentityMap($entity)
 
         if (isset($this->identityMap[$className][$idHash])) {
             if ($this->identityMap[$className][$idHash] !== $entity) {
-                throw new RuntimeException(sprintf(
-                    <<<'EXCEPTION'
+                if ($this->em->getConfiguration()->isRejectIdCollisionInIdentityMapEnabled()) {
+                    throw new RuntimeException(
+                        sprintf(
+                            <<<'EXCEPTION'
 While adding an entity of class %s with an ID hash of "%s" to the identity map,
 another object of class %s was already present for the same ID. This exception
 is a safeguard against an internal inconsistency - IDs should uniquely map to
@@ -1651,11 +1653,42 @@ public function addToIdentityMap($entity)
 
 Otherwise, it might be an ORM-internal inconsistency, please report it. 
 EXCEPTION
-                    ,
-                    get_class($entity),
-                    $idHash,
-                    get_class($this->identityMap[$className][$idHash])
-                ));
+                            ,
+                            get_class($entity),
+                            $idHash,
+                            get_class($this->identityMap[$className][$idHash])
+                        )
+                    );
+                } else {
+                    Deprecation::trigger(
+                        'doctrine/orm',
+                        'https://github.com/doctrine/orm/pull/10785',
+                        <<<'EXCEPTION'
+While adding an entity of class %s with an ID hash of "%s" to the identity map,
+another object of class %s was already present for the same ID. This will trigger
+an exception in ORM 3.0.
+
+IDs should uniquely map to entity object instances. This problem may occur if:
+
+- you use application-provided IDs and reuse ID values;
+- database-provided IDs are reassigned after truncating the database without
+clearing the EntityManager;
+- you might have been using EntityManager#getReference() to create a reference
+for a nonexistent ID that was subsequently (by the RDBMS) assigned to another
+entity. 
+
+Otherwise, it might be an ORM-internal inconsistency, please report it.
+
+To opt-in to the new exception, call 
+\Doctrine\ORM\Configuration::setRejectIdCollisionInIdentityMap on the entity
+manager's configuration.
+EXCEPTION
+                        ,
+                        get_class($entity),
+                        $idHash,
+                        get_class($this->identityMap[$className][$idHash])
+                    );
+                }
             }
 
             return false;
diff --git a/tests/Doctrine/Tests/ORM/Functional/BasicFunctionalTest.php b/tests/Doctrine/Tests/ORM/Functional/BasicFunctionalTest.php
index 7356a1bfe0..2be3feba4e 100644
--- a/tests/Doctrine/Tests/ORM/Functional/BasicFunctionalTest.php
+++ b/tests/Doctrine/Tests/ORM/Functional/BasicFunctionalTest.php
@@ -1329,6 +1329,8 @@ public function testWrongAssociationInstance(): void
 
     public function testItThrowsWhenReferenceUsesIdAssignedByDatabase(): void
     {
+        $this->_em->getConfiguration()->setRejectIdCollisionInIdentityMap(true);
+
         $user           = new CmsUser();
         $user->name     = 'test';
         $user->username = 'test';
diff --git a/tests/Doctrine/Tests/ORM/UnitOfWorkTest.php b/tests/Doctrine/Tests/ORM/UnitOfWorkTest.php
index e47af9d57d..31fa5ea8f6 100644
--- a/tests/Doctrine/Tests/ORM/UnitOfWorkTest.php
+++ b/tests/Doctrine/Tests/ORM/UnitOfWorkTest.php
@@ -927,8 +927,28 @@ public function testRemovedEntityIsRemovedFromOneToManyCollection(): void
         self::assertEmpty($user->phonenumbers->getSnapshot());
     }
 
+    public function testItTriggersADeprecationNoticeWhenApplicationProvidedIdsCollide(): void
+    {
+        // We're using application-provided IDs and assign the same ID twice
+        // Note this is about colliding IDs in the identity map in memory.
+        // Duplicate database-level IDs would be spotted when the EM is flushed.
+
+        $phone1              = new CmsPhonenumber();
+        $phone1->phonenumber = '1234';
+        $this->_unitOfWork->persist($phone1);
+
+        $phone2              = new CmsPhonenumber();
+        $phone2->phonenumber = '1234';
+
+        $this->expectDeprecationWithIdentifier('https://github.com/doctrine/orm/pull/10785');
+
+        $this->_unitOfWork->persist($phone2);
+    }
+
     public function testItThrowsWhenApplicationProvidedIdsCollide(): void
     {
+        $this->_emMock->getConfiguration()->setRejectIdCollisionInIdentityMap(true);
+
         // We're using application-provided IDs and assign the same ID twice
         // Note this is about colliding IDs in the identity map in memory.
         // Duplicate database-level IDs would be spotted when the EM is flushed.