De-duplicate union schemas

pyproject.toml

@@ -52,6 +52,7 @@ requires-python = ">=3.9"
 dependencies = [
     "avro~=1.10",
     "memoization~=0.4",
+    "more-itertools~=10.0",
     "orjson~=3.5",
     "typeguard~=4.0",
 ]

src/py_avro_schema/_schemas.py

@@ -44,6 +44,7 @@
     get_type_hints,
 )
 
+import more_itertools
 import orjson
 import typeguard
 
@@ -696,9 +697,17 @@ def __init__(self, py_type: Type[Union[Any]], namespace: Optional[str] = None, o
         args = get_args(py_type)
         self.item_schemas = [_schema_obj(arg, namespace=namespace, options=options) for arg in args]
 
-    def data(self, names: NamesType) -> JSONArray:
+    def data(self, names: NamesType) -> JSONType:
         """Return the schema data"""
-        return [schema.data(names=names) for schema in self.item_schemas]
+        # Render the item schemas
+        schemas = (item_schema.data(names=names) for item_schema in self.item_schemas)
+        # We need to deduplicate the schemas **after** rendering. This is because **different** Python types might
+        # result in the **same** Avro schema. Preserving order as order may be significant in an Avro schema.
+        unique_schemas = list(more_itertools.unique_everseen(schemas))
+        if len(unique_schemas) > 1:
+            return unique_schemas
+        else:
+            return unique_schemas[0]
 
     def sort_item_schemas(self, default_value: Any) -> None:
         """Re-order the union's schemas such that the first item corresponds with a record field's default value"""

tests/test_primitives.py

@@ -352,6 +352,18 @@ def test_union_of_union_string_int():
     assert_schema(py_type, expected)
 
 
+def test_union_str_str():
+    py_type = Union[str, str]
+    expected = "string"
+    assert_schema(py_type, expected)
+
+
+def test_union_str_annotated_str():
+    py_type = Union[str, Annotated[str, ...]]
+    expected = "string"
+    assert_schema(py_type, expected)
+
+
 def test_literal_different_types():
     py_type = Literal["", 42]
     with pytest.raises(