diff --git a/bookmarks/models.py b/bookmarks/models.py
index e65c2251..333c1b1d 100644
--- a/bookmarks/models.py
+++ b/bookmarks/models.py
@@ -168,6 +168,22 @@ def has_notes(self):
             self.instance and self.instance.notes
         )
 
+    def clean(self):
+        cleaned_data = super().clean()
+        url = cleaned_data.get("url")
+
+        if self.instance.pk and url:
+            # Ensure there is no existing Bookmark with the same URL
+            existing_bookmark = (
+                Bookmark.objects.filter(url=url, owner=self.instance.owner)
+                .exclude(pk=self.instance.pk)
+                .first()
+            )
+            if existing_bookmark:
+                self.add_error("url", "A bookmark with this URL already exists.")
+
+        return cleaned_data
+
 
 class BookmarkSearch:
     SORT_ADDED_ASC = "added_asc"
diff --git a/bookmarks/tests/test_bookmark_edit_view.py b/bookmarks/tests/test_bookmark_edit_view.py
index 18b04ec9..1ca7dba4 100644
--- a/bookmarks/tests/test_bookmark_edit_view.py
+++ b/bookmarks/tests/test_bookmark_edit_view.py
@@ -141,6 +141,42 @@ def test_should_prefill_bookmark_form_fields(self):
             html,
         )
 
+    def test_should_prevent_changing_url_to_existing_url_for_same_owner(self):
+        edited_bookmark = self.setup_bookmark(url="http://example.com/edited")
+        existing_bookmark = self.setup_bookmark(url="http://example.com/existing")
+
+        form_data = self.create_form_data(
+            {"id": edited_bookmark.id, "url": existing_bookmark.url}
+        )
+        response = self.client.post(
+            reverse("bookmarks:edit", args=[edited_bookmark.id]), form_data
+        )
+
+        self.assertEqual(response.status_code, 422)
+        self.assertInHTML(
+            "<li>A bookmark with this URL already exists.</li>",
+            response.content.decode(),
+        )
+        edited_bookmark.refresh_from_db()
+        self.assertNotEqual(edited_bookmark.url, existing_bookmark.url)
+
+    def test_should_allow_changing_url_to_existing_url_for_different_owner(self):
+        edited_bookmark = self.setup_bookmark(url="http://example.com/edited")
+        existing_bookmark = self.setup_bookmark(
+            url="http://example.com/existing", user=User.objects.create_user("other")
+        )
+
+        form_data = self.create_form_data(
+            {"id": edited_bookmark.id, "url": existing_bookmark.url}
+        )
+        response = self.client.post(
+            reverse("bookmarks:edit", args=[edited_bookmark.id]), form_data
+        )
+
+        self.assertEqual(response.status_code, 302)
+        edited_bookmark.refresh_from_db()
+        self.assertEqual(edited_bookmark.url, existing_bookmark.url)
+
     def test_should_redirect_to_return_url(self):
         bookmark = self.setup_bookmark()
         form_data = self.create_form_data()