[WEB-3285] fix: creating and updating duplicate quick links (#6557)

* fix: creating and updating duplicate quick links

* fix: improve code readibiltiy

apiserver/plane/app/serializers/workspace.py

@@ -147,6 +147,41 @@ class WorkspaceUserLinkSerializer(BaseSerializer):
         return value
 
 
+    def create(self, validated_data):
+        # Filtering the WorkspaceUserLink with the given url to check if the link already exists.
+        
+        url = validated_data.get("url")
+
+        workspace_user_link = WorkspaceUserLink.objects.filter(
+            url=url, 
+            workspace_id=validated_data.get("workspace_id"), 
+            owner=validated_data.get("owner")
+        )
+
+        if workspace_user_link.exists():
+            raise serializers.ValidationError(
+                {"error": "URL already exists for this workspace and owner"}
+            )
+        return WorkspaceUserLink.objects.create(**validated_data)
+
+    def update(self, instance, validated_data):
+        # Filtering the WorkspaceUserLink with the given url to check if the link already exists.
+
+        url = validated_data.get("url")
+
+        workspace_user_link = WorkspaceUserLink.objects.filter(
+                url=url, 
+                workspace_id=instance.workspace_id, 
+                owner=instance.owner
+            )
+
+        if workspace_user_link.exclude(pk=instance.id).exists():
+            raise serializers.ValidationError(
+                {"error": "URL already exists for this workspace and owner"}
+            )
+
+        return super().update(instance, validated_data)
+
 class IssueRecentVisitSerializer(serializers.ModelSerializer):
     project_identifier = serializers.SerializerMethodField()