diff --git a/hef/catalog.py b/hef/catalog.py
index 56aac04..0e61c54 100644
--- a/hef/catalog.py
+++ b/hef/catalog.py
@@ -126,3 +126,20 @@ def append_record(record, path) -> None:
     path = Path(path)
     with path.open("a", encoding="utf-8") as fh:
         fh.write(json.dumps(record_to_dict(record), ensure_ascii=False) + "\n")
+
+
+def index_by_id(records) -> dict:
+    """Map id -> Record, raising CatalogError on a duplicate id."""
+    idx: dict = {}
+    for r in records:
+        if r.id in idx:
+            raise CatalogError(f"duplicate record id: {r.id!r}")
+        idx[r.id] = r
+    return idx
+
+
+def validate_catalog(records) -> None:
+    """Validate every record AND cross-record invariants (currently: unique ids)."""
+    for r in records:
+        validate(r)
+    index_by_id(records)  # raises on duplicate id
diff --git a/tests/test_catalog_integrity.py b/tests/test_catalog_integrity.py
new file mode 100644
index 0000000..c12c5a3
--- /dev/null
+++ b/tests/test_catalog_integrity.py
@@ -0,0 +1,48 @@
+import pytest
+
+from hef.catalog import Record, validate_catalog, index_by_id, CatalogError
+
+
+def make_record(**o):
+    base = dict(
+        id="a",
+        title="t",
+        source_url="u",
+        source_archive="nasa",
+        license="public_domain",
+        mode="video",
+        left=0,
+        right=0,
+        dark=0,
+        light=0,
+        duration_s=1,
+        file_path="p",
+    )
+    base.update(o)
+    return Record(**base)
+
+
+def test_validate_catalog_accepts_unique_ids():
+    validate_catalog([make_record(id="a"), make_record(id="b")])  # no raise
+
+
+def test_validate_catalog_rejects_duplicate_id():
+    with pytest.raises(CatalogError) as e:
+        validate_catalog([make_record(id="dup"), make_record(id="dup")])
+    assert "dup" in str(e.value)
+
+
+def test_validate_catalog_validates_each_record():
+    with pytest.raises(CatalogError):
+        validate_catalog([make_record(left=9)])
+
+
+def test_index_by_id_round_trips():
+    recs = [make_record(id="a"), make_record(id="b")]
+    idx = index_by_id(recs)
+    assert idx["a"].id == "a" and idx["b"].id == "b"
+
+
+def test_index_by_id_rejects_duplicates():
+    with pytest.raises(CatalogError):
+        index_by_id([make_record(id="x"), make_record(id="x")])