diff --git a/masque/file/oasis.py b/masque/file/oasis.py
index 5e343ea..95a2039 100644
--- a/masque/file/oasis.py
+++ b/masque/file/oasis.py
@@ -714,7 +714,7 @@ def properties_to_annotations(
                 string = repr(value)
                 logger.warning(f'Converting property value for key ({key}) to string ({string})')
                 values.append(string)
-        annotations[key] = values
+        annotations.setdefault(key, []).extend(values)
     return annotations
 
 
diff --git a/masque/test/test_oasis.py b/masque/test/test_oasis.py
index b1129f4..ad9bbf9 100644
--- a/masque/test/test_oasis.py
+++ b/masque/test/test_oasis.py
@@ -4,6 +4,8 @@ from numpy.testing import assert_equal
 
 from ..pattern import Pattern
 from ..library import Library
+
+
 def test_oasis_roundtrip(tmp_path: Path) -> None:
     # Skip if fatamorgana is not installed
     pytest.importorskip("fatamorgana")
@@ -23,3 +25,20 @@ def test_oasis_roundtrip(tmp_path: Path) -> None:
 
     # Check bounds
     assert_equal(read_lib["cell1"].get_bounds(), [[0, 0], [10, 10]])
+
+
+def test_oasis_properties_to_annotations_merges_repeated_keys() -> None:
+    pytest.importorskip("fatamorgana")
+    import fatamorgana.records as fatrec
+    from ..file.oasis import properties_to_annotations
+
+    annotations = properties_to_annotations(
+        [
+            fatrec.Property("k", [1], is_standard=False),
+            fatrec.Property("k", [2, 3], is_standard=False),
+        ],
+        {},
+        {},
+    )
+
+    assert annotations == {"k": [1, 2, 3]}