pythongh-107609: Fix duplicate module check in Argument Clinic

Author: erlend-aasland

Lib/test/test_clinic.py

@@ -416,6 +416,16 @@ def __init__(self):
         """
         self.expect_failure(block, err, lineno=8)
 
+    def test_module_already_got_one(self):
+        err = "Already defined module 'm'!"
+        block = """
+            /*[clinic input]
+            module m
+            module m
+            [clinic start generated code]*/
+        """
+        self.expect_failure(block, err, lineno=3)
+
 
 class ClinicGroupPermuterTest(TestCase):
     def _test(self, l, m, r, output):

Misc/NEWS.d/next/Tools-Demos/2023-08-04-00-04-40.gh-issue-107609.2DqgtL.rst

@@ -0,0 +1,3 @@
+Fix duplicate module check in Argument Clinic. Previously, a duplicate
+definition would incorrectly be silently accepted. Patch by Erlend E.
+Aasland.

Tools/clinic/clinic.py

@@ -4478,7 +4478,7 @@ def directive_module(self, name: str) -> None:
         if cls:
             fail("Can't nest a module inside a class!")
 
-        if name in module.classes:
+        if name in module.modules:
             fail("Already defined module " + repr(name) + "!")
 
         m = Module(name, module)