Do not emit type arguments twice

Fixes sphinx-docgh-7567

sphinx/util/typing.py

@@ -75,8 +75,13 @@ def _stringify_py37(annotation: Any) -> str:
             qualname = stringify(annotation.__origin__)  # ex. Union
     elif hasattr(annotation, '__qualname__'):
         qualname = '%s.%s' % (module, annotation.__qualname__)
+    elif hasattr(annotation, '__origin__'):
+        # instantiated generic provided by a user
+        qualname = stringify(annotation.__origin__)
     else:
-        qualname = repr(annotation)
+        # we weren't able to extract the base type, appending arguments would
+        # only make them appear twice
+        return repr(annotation)
 
     if getattr(annotation, '__args__', None):
         if qualname == 'Union':
@@ -91,7 +96,7 @@ def _stringify_py37(annotation: Any) -> str:
             return '%s[[%s], %s]' % (qualname, args, returns)
         elif str(annotation).startswith('typing.Annotated'):  # for py39+
             return stringify(annotation.__args__[0])
-        elif annotation._special:
+        elif getattr(annotation, '_special', False):
             return qualname
         else:
             args = ', '.join(stringify(a) for a in annotation.__args__)

tests/test_util_typing.py

@@ -10,7 +10,7 @@
 
 import sys
 from numbers import Integral
-from typing import Any, Dict, List, TypeVar, Union, Callable, Tuple, Optional
+from typing import Any, Dict, List, TypeVar, Union, Callable, Tuple, Optional, Generic
 
 import pytest
 
@@ -24,6 +24,11 @@ class MyClass1:
 class MyClass2(MyClass1):
     __qualname__ = '<MyClass2>'
 
+T = TypeVar('T')
+
+class MyList(List[T]):
+    pass
+
 
 def test_stringify():
     assert stringify(int) == "int"
@@ -42,6 +47,7 @@ def test_stringify_type_hints_containers():
     assert stringify(Tuple[str, str, str]) == "Tuple[str, str, str]"
     assert stringify(Tuple[str, ...]) == "Tuple[str, ...]"
     assert stringify(List[Dict[str, Tuple]]) == "List[Dict[str, Tuple]]"
+    assert stringify(MyList[Tuple[int, int]]) == "test_util_typing.MyList[Tuple[int, int]]"
 
 
 @pytest.mark.skipif(sys.version_info < (3, 9), reason='python 3.9+ is required.')