fix(checker): handle already defined types

midas/checker/midas.py

@@ -170,7 +170,10 @@ class MidasTyper(m.Stmt.Visitor[None], m.Expr.Visitor[Type], m.Type.Visitor[Type
             type = GenericType(name=name, params=params, body=type)
         else:
             type = DerivedType(name=name, type=type)
+        try:
             self.types.define_type(name, type)
+        except ValueError:
+            self.reporter.error(stmt.location, f"Type {name} already defined")
         self._local_variables.clear()
         self._current_name = None
 
@@ -178,7 +181,10 @@ class MidasTyper(m.Stmt.Visitor[None], m.Expr.Visitor[Type], m.Type.Visitor[Type
         name: str = stmt.name.lexeme
         self._current_name = name
         type: Type = stmt.type.accept(self)
+        try:
             self.types.define_type(name, type)
+        except ValueError:
+            self.reporter.error(stmt.location, f"Type {name} already defined")
         self._current_name = None
 
     def visit_member_stmt(self, stmt: m.MemberStmt) -> None: ...

midas/checker/python.py

@@ -1111,7 +1111,13 @@ class PythonTyper(
                     else:
                         variance = Variance.CONTRAVARIANT
                 var: TypeVar = TypeVar(name=name, bound=bound, variance=variance)
+                try:
                     self.types.define_type(name, var)
+                except ValueError:
+                    self.reporter.error(
+                        call.location,
+                        f"A type or type variable with the name {name} is already defined",
+                    )
                 return var
 
             case _: