From 9375e2b27e27ce6ce5ca28128b0403eb894d3553 Mon Sep 17 00:00:00 2001
From: Marc Mueller <30130371+cdce8p@users.noreply.github.com>
Date: Sun, 21 Dec 2025 23:28:26 +0100
Subject: [PATCH] Add test case for duplicate keyword attributes

---
 Lib/test/test_syntax.py | 6 ++++++
 Python/ceval.c          | 3 ++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/Lib/test/test_syntax.py b/Lib/test/test_syntax.py
index 93f0b98de71..f245db287be 100644
--- a/Lib/test/test_syntax.py
+++ b/Lib/test/test_syntax.py
@@ -2345,6 +2345,12 @@ Invalid pattern matching constructs:
     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, ...):
     ...     ...
diff --git a/Python/ceval.c b/Python/ceval.c
index 51f9fcde6d0..2623eff5a33 100644
--- a/Python/ceval.c
+++ b/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) {