diff --git a/src/paperless_ai/tests/test_vector_store.py b/src/paperless_ai/tests/test_vector_store.py
index 88ffce31b..641cafc57 100644
--- a/src/paperless_ai/tests/test_vector_store.py
+++ b/src/paperless_ai/tests/test_vector_store.py
@@ -302,3 +302,19 @@ class TestPaperlessLanceVectorStoreMaintenance:
         )
         assert doc1 == ["1-0"]
         assert table.count_rows() == 2
+
+    def test_ensure_scalar_index_is_idempotent(
+        self,
+        store: PaperlessLanceVectorStore,
+    ) -> None:
+        store.add([_node("1-0", "1", "text", 0.5)])
+        store.ensure_document_id_scalar_index()
+        # Second call must not raise and must not replace the existing index.
+        store.ensure_document_id_scalar_index()
+        assert store._has_scalar_index()
+
+    def test_ensure_scalar_index_noop_on_empty_store(
+        self,
+        store: PaperlessLanceVectorStore,
+    ) -> None:
+        store.ensure_document_id_scalar_index()  # no table yet — must not raise
diff --git a/src/paperless_ai/vector_store.py b/src/paperless_ai/vector_store.py
index ffbfe4969..7ab6a4dbf 100644
--- a/src/paperless_ai/vector_store.py
+++ b/src/paperless_ai/vector_store.py
@@ -239,6 +239,9 @@ class PaperlessLanceVectorStore(BasePydanticVectorStore):
     def _has_vector_index(self) -> bool:
         return any("vector" in idx.columns for idx in self._table.list_indices())
 
+    def _has_scalar_index(self) -> bool:
+        return any("document_id" in idx.columns for idx in self._table.list_indices())
+
     def maybe_create_ann_index(self, min_rows: int = ANN_INDEX_MIN_ROWS) -> None:
         """Best-effort: build an IVF index once the table is large enough.
 
@@ -273,11 +276,14 @@ class PaperlessLanceVectorStore(BasePydanticVectorStore):
 
     def ensure_document_id_scalar_index(self) -> None:
         """Create a scalar index on the filter column (never on the merge key
-        ``id`` — see LanceDB #3177)."""
+        ``id`` — see https://github.com/lancedb/lancedb/issues/3177).
+        No-op if the index already exists."""
         if self._table is None:
             return
+        if self._has_scalar_index():
+            return
         try:
-            self._table.create_scalar_index("document_id", replace=True)
+            self._table.create_scalar_index("document_id")
         except Exception as e:  # pragma: no cover
             logger.warning("Skipping document_id scalar index: %s", e)