diff --git a/app/Http/Controllers/Api/ProjectInvitationController.php b/app/Http/Controllers/Api/ProjectInvitationController.php
index 38344e0a3..afa1e1f3b 100644
--- a/app/Http/Controllers/Api/ProjectInvitationController.php
+++ b/app/Http/Controllers/Api/ProjectInvitationController.php
@@ -83,7 +83,7 @@ public function join(JoinProjectInvitation $request, $id)
                 if ($invitation->add_to_sessions) {
                     AnnotationSession::join('project_volume', 'annotation_sessions.volume_id', '=', 'project_volume.volume_id')
                         ->where('project_volume.project_id', $project->id)
-                        ->eachById(fn ($session) => $session->users()->attach($userId));
+                        ->eachById(fn ($session) => $session->users()->syncWithoutDetaching([$userId]));
                 }
             }
 
diff --git a/tests/php/Http/Controllers/Api/ProjectInvitationControllerTest.php b/tests/php/Http/Controllers/Api/ProjectInvitationControllerTest.php
index 9ab75313c..b654083f6 100644
--- a/tests/php/Http/Controllers/Api/ProjectInvitationControllerTest.php
+++ b/tests/php/Http/Controllers/Api/ProjectInvitationControllerTest.php
@@ -259,6 +259,35 @@ public function testJoinAddToSessions()
         $this->assertNotNull($session->users()->find($this->user()->id));
     }
 
+    public function testJoinAddToSessionsAlreadyExist()
+    {
+        $session = AnnotationSession::factory()->create([
+            'volume_id' => $this->volume()->id,
+            'starts_at' => '2023-11-04',
+            'ends_at' => '2023-11-05',
+        ]);
+
+        $invitation = ProjectInvitation::factory()->create([
+            'project_id' => $this->project()->id,
+            'role_id' => Role::editorId(),
+            'add_to_sessions' => true,
+        ]);
+
+        $session->users()->attach($this->user());
+        $session->users()->attach($this->expert());
+
+        $this->beUser();
+        $this
+            ->postJson("/api/v1/project-invitations/{$invitation->id}/join", [
+                'token' => $invitation->uuid,
+            ])
+            ->assertSuccessful();
+
+        $this->assertNotNull($session->users()->find($this->user()->id));
+        // The expert should not be detached by this.
+        $this->assertNotNull($session->users()->find($this->expert()->id));
+    }
+
     public function testShowQrCode()
     {
         $invitation = ProjectInvitation::factory()->create([