Add test case for duplicate keyword attributes

Author: cdce8p

Lib/test/test_syntax.py

@@ -2345,6 +2345,12 @@
     Traceback (most recent call last):
     SyntaxError: positional patterns follow keyword patterns
 
+    >>> match ...:
+    ...   case Foo(y=1, x=2, y=3):
+    ...     ...
+    Traceback (most recent call last):
+    SyntaxError: attribute name repeated in class pattern: y
+
     >>> match ...:
     ...   case C(a=b, c, d=e, f, g=h, i, j=k, ...):
     ...     ...

Python/ceval.c

@@ -887,7 +887,8 @@ _PyEval_MatchClass(PyThreadState *tstate, PyObject *subject, PyObject *type,
     // So far so good:
     PyObject *seen = NULL;
     // Only check for duplicates if there is at least one positional attribute
-    // and two or more attributes in total.
+    // and two or more attributes in total. Duplicate keyword attributes are
+    // detected during the compile stage and raise a SyntaxError.
     if (nargs > 0 && nattrs > 1) {
         seen = PySet_New(NULL);
         if (seen == NULL) {