diff --git a/app/features/registry/service.py b/app/features/registry/service.py
index 515f17ca..71243ae3 100644
--- a/app/features/registry/service.py
+++ b/app/features/registry/service.py
@@ -601,18 +601,30 @@ async def _find_duplicate(
             data_window_end: Data window end date.
 
         Returns:
-            Existing run or None.
+            The most recent matching run, or None.
+
+        Note:
+            Under ``registry_duplicate_policy="detect"`` (the default), duplicate
+            runs are intentionally created, so multiple non-archived rows may
+            share the same config hash. This query therefore orders by
+            ``created_at`` and takes the first row rather than asserting a single
+            match — ``scalar_one_or_none()`` would raise ``MultipleResultsFound``.
         """
-        stmt = select(ModelRun).where(
-            (ModelRun.config_hash == config_hash)
-            & (ModelRun.store_id == store_id)
-            & (ModelRun.product_id == product_id)
-            & (ModelRun.data_window_start == data_window_start)
-            & (ModelRun.data_window_end == data_window_end)
-            & (ModelRun.status != RunStatusORM.ARCHIVED.value)
+        stmt = (
+            select(ModelRun)
+            .where(
+                (ModelRun.config_hash == config_hash)
+                & (ModelRun.store_id == store_id)
+                & (ModelRun.product_id == product_id)
+                & (ModelRun.data_window_start == data_window_start)
+                & (ModelRun.data_window_end == data_window_end)
+                & (ModelRun.status != RunStatusORM.ARCHIVED.value)
+            )
+            .order_by(ModelRun.created_at.desc())
+            .limit(1)
         )
         result = await db.execute(stmt)
-        return result.scalar_one_or_none()
+        return result.scalars().first()
 
     def _model_to_response(self, model_run: ModelRun) -> RunResponse:
         """Convert ORM model to response schema.
diff --git a/app/features/registry/tests/test_routes.py b/app/features/registry/tests/test_routes.py
index 72d889f1..0b9e2e5e 100644
--- a/app/features/registry/tests/test_routes.py
+++ b/app/features/registry/tests/test_routes.py
@@ -91,6 +91,30 @@ async def test_create_run_invalid_date_order(self, client: AsyncClient) -> None:
         )
         assert response.status_code == 422
 
+    async def test_create_run_repeated_duplicate_does_not_500(self, client: AsyncClient) -> None:
+        """Repeated identical runs must not 500 (regression for #146).
+
+        Under the default ``registry_duplicate_policy="detect"`` duplicate runs
+        are created intentionally, so multiple non-archived rows can share one
+        config hash. ``_find_duplicate`` previously used ``scalar_one_or_none()``,
+        which raised ``MultipleResultsFound`` once two duplicates existed — the
+        third POST returned ``HTTP 500 Database Error``.
+        """
+        payload = {
+            "model_type": "test-dup-regression",
+            "model_config": {"strategy": "last_value"},
+            "data_window_start": "2024-01-01",
+            "data_window_end": "2024-03-31",
+            "store_id": 1,
+            "product_id": 1,
+        }
+
+        # Three identical creates: 1st has no prior match, 2nd has one,
+        # 3rd would hit the MultipleResultsFound trap before the fix.
+        for _ in range(3):
+            response = await client.post("/registry/runs", json=payload)
+            assert response.status_code == 201, response.text
+
 
 class TestListRunsEndpoint:
     """Tests for GET /registry/runs endpoint."""