Ignore duplicates in join_type_list

Author: sterliakov

mypy/join.py

@@ -885,8 +885,9 @@ def join_type_list(types: Sequence[Type]) -> Type:
         # with all variable length tuples, and this makes it possible.
         return UninhabitedType()
     joined = types[0]
-    for t in types[1:]:
-        joined = join_types(joined, t)
+    for i, t in enumerate(types[1:], 1):
+        if t not in types[:i]:
+            joined = join_types(joined, t)
     return joined