diff --git a/codex/ingest.py b/codex/ingest.py
index ef5cb82..a6d1663 100644
--- a/codex/ingest.py
+++ b/codex/ingest.py
@@ -164,10 +164,26 @@ def ingest_paper(
                 )
                 break
             except psycopg.errors.UniqueViolation as exc:
-                if base_bibkey is None or "bibkey" not in str(exc).lower() or attempt == 26:
-                    raise
+                msg = str(exc).lower()
                 conn.rollback()
+                if "openalex" in msg:
+                    # Two different papers.id claim the same OpenAlex work — a real data
+                    # conflict, not something to auto-rename. Surface it clearly instead
+                    # of the raw psycopg error (companion gap to C-15's bibkey case).
+                    raise ValueError(
+                        f"openalex_id {paper.openalex_id!r} is already attached to a "
+                        f"different paper; ingesting '{paper.id}' would duplicate it — "
+                        "resolve the conflicting paper first."
+                    ) from exc
+                if base_bibkey is None or "bibkey" not in msg or attempt == 26:
+                    raise
                 paper.bibkey = f"{base_bibkey}{chr(ord('a') + attempt)}"
+                logger.warning(
+                    "bibkey %r already exists; ingesting '%s' as %r instead",
+                    base_bibkey,
+                    paper.id,
+                    paper.bibkey,
+                )
 
         # Register openalex_id in paper_identifiers (alias table).
         # Every ingest records the primary openalex_id so the graph JOIN
diff --git a/tests/ingest/test_ingest.py b/tests/ingest/test_ingest.py
index 0f80f45..95f4423 100644
--- a/tests/ingest/test_ingest.py
+++ b/tests/ingest/test_ingest.py
@@ -560,6 +560,35 @@ def test_ingest_disambiguates_bibkey_on_unique_violation() -> None:
     mock_conn.rollback.assert_called_once()
 
 
+def test_ingest_openalex_collision_raises_clear_error() -> None:
+    """An openalex_id UNIQUE collision raises a clear, actionable error (audit C-15).
+
+    Unlike a bibkey clash (auto-suffixed), a shared openalex_id means two papers
+    claim the same OpenAlex work — a real data conflict that must surface, not the
+    raw psycopg error.
+    """
+    paper = _make_paper()
+    mock_conn = MagicMock()
+
+    def execute_side_effect(sql: str, params: Any = None) -> MagicMock:
+        if "INSERT INTO papers" in sql:
+            raise psycopg.errors.UniqueViolation(
+                'duplicate key value violates unique constraint "papers_openalex_id_key"'
+            )
+        return MagicMock()
+
+    mock_conn.execute.side_effect = execute_side_effect
+
+    with (
+        patch("codex.ingest.openalex.fetch_paper", return_value=paper),
+        patch("codex.ingest.openalex.fetch_citations", return_value=[]),
+        patch("codex.ingest.get_embedder", return_value=_fake_embedder()),
+        patch("codex.ingest.get_conn", side_effect=_make_conn_cm(mock_conn)),
+        pytest.raises(ValueError, match="openalex_id"),
+    ):
+        ingest_paper(paper.id)
+
+
 # ---------------------------------------------------------------------------