Check duplicate bases when defining TypedDict (#11485)

### Description

Closes #3673

Adds a trivial for-loop and a helper set to check duplicates of `typeddict_bases`.

## Test Plan

Adds a new test case `testCannotCreateTypedDictWithDuplicateBases`

Author: 97littleleaf11

mypy/semanal_typeddict.py

@@ -67,16 +67,30 @@ def analyze_typeddict_classdef(self, defn: ClassDef) -> Tuple[bool, Optional[Typ
                 defn.analyzed.line = defn.line
                 defn.analyzed.column = defn.column
                 return True, info
+
             # Extending/merging existing TypedDicts
-            if any(not isinstance(expr, RefExpr) or
-                   expr.fullname not in TPDICT_NAMES and
-                   not self.is_typeddict(expr) for expr in defn.base_type_exprs):
-                self.fail("All bases of a new TypedDict must be TypedDict types", defn)
-            typeddict_bases = list(filter(self.is_typeddict, defn.base_type_exprs))
+            typeddict_bases = []
+            typeddict_bases_set = set()
+            for expr in defn.base_type_exprs:
+                if isinstance(expr, RefExpr) and expr.fullname in TPDICT_NAMES:
+                    if 'TypedDict' not in typeddict_bases_set:
+                        typeddict_bases_set.add('TypedDict')
+                    else:
+                        self.fail('Duplicate base class "TypedDict"', defn)
+                elif isinstance(expr, RefExpr) and self.is_typeddict(expr):
+                    assert expr.fullname
+                    if expr.fullname not in typeddict_bases_set:
+                        typeddict_bases_set.add(expr.fullname)
+                        typeddict_bases.append(expr)
+                    else:
+                        assert isinstance(expr.node, TypeInfo)
+                        self.fail('Duplicate base class "%s"' % expr.node.name, defn)
+                else:
+                    self.fail("All bases of a new TypedDict must be TypedDict types", defn)
+
             keys: List[str] = []
             types = []
             required_keys = set()
-
             # Iterate over bases in reverse order so that leftmost base class' keys take precedence
             for base in reversed(typeddict_bases):
                 assert isinstance(base, RefExpr)
@@ -328,3 +342,6 @@ def is_typeddict(self, expr: Expression) -> bool:
 
     def fail(self, msg: str, ctx: Context, *, code: Optional[ErrorCode] = None) -> None:
         self.api.fail(msg, ctx, code=code)
+
+    def note(self, msg: str, ctx: Context) -> None:
+        self.api.note(msg, ctx)

test-data/unit/check-typeddict.test

@@ -171,6 +171,21 @@ p: Point2D
 reveal_type(p)  # N: Revealed type is "TypedDict('__main__.Point2D', {'x': builtins.int, 'y': builtins.int})"
 [builtins fixtures/dict.pyi]
 
+[case testCannotCreateTypedDictWithDuplicateBases]
+# https://github.com/python/mypy/issues/3673
+from typing import TypedDict
+
+class A(TypedDict):
+    x: str
+    y: int
+
+class B(A, A): # E: Duplicate base class "A"
+    z: str
+
+class C(TypedDict, TypedDict): # E: Duplicate base class "TypedDict"
+    c1: int
+[typing fixtures/typing-typeddict.pyi]
+
 [case testCannotCreateTypedDictWithClassWithOtherStuff]
 # flags: --python-version 3.6
 from mypy_extensions import TypedDict