Detect redefinitions of type parameters

mypy/semanal.py

@@ -1664,10 +1664,23 @@ def push_type_args(
             tvs.append((p.name, tv))
 
         for name, tv in tvs:
-            self.add_symbol(name, tv, context, no_progress=True, type_param=True)
+            if self.is_defined_type_param(name):
+                self.fail(f'"{name}" already defined as a type parameter', context)
+            else:
+                self.add_symbol(name, tv, context, no_progress=True, type_param=True)
 
         return tvs
 
+    def is_defined_type_param(self, name: str) -> bool:
+        for names in self.locals:
+            if names is None:
+                continue
+            if name in names:
+                node = names[name].node
+                if isinstance(node, TypeVarLikeExpr):
+                    return True
+        return False
+
     def analyze_type_param(self, type_param: TypeParam) -> TypeVarLikeExpr | None:
         fullname = self.qualified_name(type_param.name)
         if type_param.upper_bound:

test-data/unit/check-python312.test

@@ -1137,3 +1137,15 @@ class Outer:
     # Likewise, 'Inner' would not be available in these type annotations.
     def method1[T](self, a: Inner[T]) -> Inner[T]:
         return a
+
+[case testPEP695RedefineTypeParameterInScope]
+# mypy: enable-incomplete-feature=NewGenericSyntax
+class C[T]:
+    def m[T](self, x: T) -> T:  # E: "T" already defined as a type parameter
+        return x
+    def m2(self) -> None:
+        def nested[T](x: T) -> T:  # E: "T" already defined as a type parameter
+            return x
+
+def f[S, S](x: S) -> S:  # E: "S" already defined as a type parameter
+    return x