diff --git a/.github/workflows/continuous-integration.yml b/.github/workflows/continuous-integration.yml
index b0b60532f7a..b90bf61646a 100644
--- a/.github/workflows/continuous-integration.yml
+++ b/.github/workflows/continuous-integration.yml
@@ -39,6 +39,7 @@ jobs:
         php-version:
           - "8.4"
           - "8.5"
+          - "8.6"
         dbal-version:
           - "default"
         extension:
@@ -190,6 +191,7 @@ jobs:
         php-version:
           - "8.4"
           - "8.5"
+          - "8.6"
         dbal-version:
           - "default"
         postgres-version:
@@ -270,6 +272,7 @@ jobs:
         php-version:
           - "8.4"
           - "8.5"
+          - "8.6"
         dbal-version:
           - "default"
           - "4@dev"
@@ -343,6 +346,7 @@ jobs:
         php-version:
           - "8.4"
           - "8.5"
+          - "8.6"
         dbal-version:
           - "default"
         mysql-version:
diff --git a/docs/en/tutorials/extra-lazy-associations.rst b/docs/en/tutorials/extra-lazy-associations.rst
index e6e6f14b527..3bf225cf65b 100644
--- a/docs/en/tutorials/extra-lazy-associations.rst
+++ b/docs/en/tutorials/extra-lazy-associations.rst
@@ -15,10 +15,10 @@ the first time its accessed. If you mark an association as extra lazy the follow
 can be called without triggering a full load of the collection:
 
 -  ``Collection#contains($entity)``
--  ``Collection#containsKey($key)``
+-  ``Collection#containsKey($key)`` only if the association includes an ``indexBy`` field.
 -  ``Collection#count()``
 -  ``Collection#first()``
--  ``Collection#get($key)``
+-  ``Collection#get($key)`` only if the association includes an ``indexBy`` field.
 -  ``Collection#isEmpty()``
 -  ``Collection#slice($offset, $length = null)``
 
diff --git a/src/UnitOfWork.php b/src/UnitOfWork.php
index 00017cff82a..2aa01696f94 100644
--- a/src/UnitOfWork.php
+++ b/src/UnitOfWork.php
@@ -2719,6 +2719,10 @@ public function createEntity(string $className, array $data, array &$hints = [])
                             // is foreign key composite
                             && ! ($targetClass->hasAssociation($assoc->mappedBy) && count($targetClass->getAssociationMapping($assoc->mappedBy)->joinColumns) > 1)
                             && ! $assoc->isIndexed()
+                            // Loading from the second-level cache does not go through ObjectHydrator,
+                            // so deferred batch eager loads are never flushed there. Load immediately
+                            // instead (also allows hitting the collection cache region).
+                            && ! isset($hints[Query::HINT_CACHE_ENABLED])
                         ) {
                             $this->scheduleCollectionForBatchLoading($pColl, $class);
                         } else {
diff --git a/src/Utility/IdentifierFlattener.php b/src/Utility/IdentifierFlattener.php
index 2f16e56e4ac..9ba08303c35 100644
--- a/src/Utility/IdentifierFlattener.php
+++ b/src/Utility/IdentifierFlattener.php
@@ -12,6 +12,7 @@
 use function assert;
 use function implode;
 use function is_a;
+use function is_object;
 
 /**
  * The IdentifierFlattener utility now houses some of the identifier manipulation logic from unit of work, so that it
@@ -47,7 +48,7 @@ public function flattenIdentifier(ClassMetadata $class, array $id): array
         $flatId = [];
 
         foreach ($class->identifier as $field) {
-            if (isset($class->associationMappings[$field]) && isset($id[$field]) && is_a($id[$field], $class->associationMappings[$field]->targetEntity)) {
+            if (isset($class->associationMappings[$field]) && isset($id[$field]) && is_object($id[$field]) && is_a($id[$field], $class->associationMappings[$field]->targetEntity)) {
                 $targetClassMetadata = $this->metadataFactory->getMetadataFor(
                     $class->associationMappings[$field]->targetEntity,
                 );
diff --git a/tests/Tests/ORM/Functional/Ticket/EagerSecondLevelCacheCollectionTest.php b/tests/Tests/ORM/Functional/Ticket/EagerSecondLevelCacheCollectionTest.php
new file mode 100644
index 00000000000..8f1f5ba9e00
--- /dev/null
+++ b/tests/Tests/ORM/Functional/Ticket/EagerSecondLevelCacheCollectionTest.php
@@ -0,0 +1,140 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\Tests\ORM\Functional\Ticket;
+
+use Doctrine\Common\Collections\ArrayCollection;
+use Doctrine\Common\Collections\Collection;
+use Doctrine\ORM\Mapping\Cache;
+use Doctrine\ORM\Mapping\Column;
+use Doctrine\ORM\Mapping\Entity;
+use Doctrine\ORM\Mapping\GeneratedValue;
+use Doctrine\ORM\Mapping\Id;
+use Doctrine\ORM\Mapping\JoinColumn;
+use Doctrine\ORM\Mapping\ManyToOne;
+use Doctrine\ORM\Mapping\OneToMany;
+use Doctrine\Tests\OrmFunctionalTestCase;
+
+/**
+ * Reproduces duplicate hydration of EAGER OneToMany collections when the owner is loaded from the
+ * second-level cache, the collection is initialized (e.g. via the collection cache), and a later
+ * hydration triggers deferred eager collection loading.
+ */
+final class EagerSecondLevelCacheCollectionTest extends OrmFunctionalTestCase
+{
+    protected function setUp(): void
+    {
+        $this->enableSecondLevelCache();
+
+        parent::setUp();
+
+        $this->createSchemaForModels(
+            EagerSlcUser::class,
+            EagerSlcRole::class,
+            EagerSlcOther::class,
+        );
+    }
+
+    public function testEagerCollectionIsNotDuplicatedAfterSecondLevelCacheHit(): void
+    {
+        $user = new EagerSlcUser('alice');
+        $user->addRole(new EagerSlcRole($user, 'ROLE_USER'));
+        $user->addRole(new EagerSlcRole($user, 'ROLE_ADMIN'));
+        $other = new EagerSlcOther('marker');
+
+        $this->_em->persist($user);
+        $this->_em->persist($other);
+        $this->_em->flush();
+
+        $userId  = $user->id;
+        $otherId = $other->id;
+
+        $this->_em->clear();
+
+        // Warm entity + collection cache (first load misses, then puts).
+        $warmed = $this->_em->find(EagerSlcUser::class, $userId);
+        self::assertNotNull($warmed);
+        self::assertCount(2, $warmed->roles);
+        $this->_em->clear();
+
+        // Load owner from the entity second-level cache. EAGER schedules deferred collection
+        // loading, but DefaultEntityHydrator does not call triggerEagerLoads().
+        $cachedUser = $this->_em->find(EagerSlcUser::class, $userId);
+        self::assertNotNull($cachedUser);
+
+        // Initialize the collection (collection-cache hit or DB) before deferred eager runs.
+        self::assertCount(2, $cachedUser->roles);
+
+        // A subsequent hydration flushes pending deferred eager collection loads.
+        $this->_em->find(EagerSlcOther::class, $otherId);
+
+        self::assertCount(
+            2,
+            $cachedUser->roles,
+            'EAGER OneToMany must not be hydrated twice after a second-level cache hit',
+        );
+    }
+}
+
+#[Entity]
+#[Cache(usage: 'NONSTRICT_READ_WRITE')]
+class EagerSlcUser
+{
+    #[Id]
+    #[Column(type: 'integer')]
+    #[GeneratedValue]
+    public int|null $id = null;
+
+    /** @var Collection<int, EagerSlcRole> */
+    #[Cache(usage: 'NONSTRICT_READ_WRITE')]
+    #[OneToMany(targetEntity: EagerSlcRole::class, mappedBy: 'user', cascade: ['persist'], fetch: 'EAGER')]
+    public Collection $roles;
+
+    public function __construct(
+        #[Column(length: 255)]
+        public string $name,
+    ) {
+        $this->roles = new ArrayCollection();
+    }
+
+    public function addRole(EagerSlcRole $role): void
+    {
+        $this->roles->add($role);
+    }
+}
+
+#[Entity]
+#[Cache(usage: 'NONSTRICT_READ_WRITE')]
+class EagerSlcRole
+{
+    #[Id]
+    #[Column(type: 'integer')]
+    #[GeneratedValue]
+    public int|null $id = null;
+
+    public function __construct(
+        #[Cache(usage: 'NONSTRICT_READ_WRITE')]
+        #[ManyToOne(targetEntity: EagerSlcUser::class, inversedBy: 'roles')]
+        #[JoinColumn(nullable: false)]
+        public EagerSlcUser $user,
+        #[Column(length: 255)]
+        public string $name,
+    ) {
+    }
+}
+
+#[Entity]
+class EagerSlcOther
+{
+    #[Id]
+    #[Column(type: 'integer')]
+    #[GeneratedValue]
+    public int|null $id = null;
+
+    public function __construct(
+        #[Column(length: 255)]
+        public string $name,
+    ) {
+    }
+}