diff --git a/.changes/unreleased/Fixed-20241205-133702.yaml b/.changes/unreleased/Fixed-20241205-133702.yaml
new file mode 100644
index 000000000..9fcafdb9f
--- /dev/null
+++ b/.changes/unreleased/Fixed-20241205-133702.yaml
@@ -0,0 +1,5 @@
+kind: Fixed
+body: Enforce unique contraint on activity storedobject
+time: 2024-12-05T13:37:02.635977414+01:00
+custom:
+    Issue: "337"
diff --git a/src/Bundle/ChillActivityBundle/Entity/Activity.php b/src/Bundle/ChillActivityBundle/Entity/Activity.php
index 4701d2483..408c27ed8 100644
--- a/src/Bundle/ChillActivityBundle/Entity/Activity.php
+++ b/src/Bundle/ChillActivityBundle/Entity/Activity.php
@@ -84,6 +84,11 @@ class Activity implements AccompanyingPeriodLinkedWithSocialIssuesEntityInterfac
      */
     #[Assert\Valid(traverse: true)]
     #[ORM\ManyToMany(targetEntity: StoredObject::class, cascade: ['persist'])]
+    #[ORM\JoinTable(
+        name: 'activity_storedobject',
+        joinColumns: new ORM\JoinColumn(name: 'activity_id', referencedColumnName: 'id', nullable: false),
+        inverseJoinColumns: new ORM\InverseJoinColumn(name: 'storedobject_id', referencedColumnName: 'id', unique: true, nullable: false)
+    )]
     private Collection $documents;
 
     #[ORM\Column(type: \Doctrine\DBAL\Types\Types::TIME_MUTABLE, nullable: true)]
diff --git a/src/Bundle/ChillActivityBundle/migrations/Version20241202173942.php b/src/Bundle/ChillActivityBundle/migrations/Version20241202173942.php
new file mode 100644
index 000000000..03ea103dc
--- /dev/null
+++ b/src/Bundle/ChillActivityBundle/migrations/Version20241202173942.php
@@ -0,0 +1,46 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * Chill is a software for social workers
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+namespace Chill\Migrations\Activity;
+
+use Doctrine\DBAL\Schema\Schema;
+use Doctrine\Migrations\AbstractMigration;
+
+final class Version20241202173942 extends AbstractMigration
+{
+    public function getDescription(): string
+    {
+        return 'Add a unique constraint on the storedobject linked to an activity';
+    }
+
+    public function up(Schema $schema): void
+    {
+        $this->addSql(
+            'WITH duplicate_activities AS (
+                SELECT storedobject_id, array_agg(activity_id ORDER BY activity_id DESC) AS activities
+                FROM activity_storedobject
+                GROUP BY storedobject_id
+                HAVING count(*) > 1
+            )
+            DELETE FROM activity_storedobject
+            WHERE activity_id IN (
+                SELECT unnest(activities[2:]) -- Keep the highest ID, delete the rest
+                FROM duplicate_activities
+            );'
+        );
+        $this->addSql('CREATE UNIQUE INDEX unique_storedobject_id ON activity_storedobject (storedobject_id)');
+    }
+
+    public function down(Schema $schema): void
+    {
+        $this->addSql('DROP INDEX IF EXISTS unique_storedobject_id');
+    }
+}