gh-159792: Suppress once-filtered warnings across modules

Lib/test/test_warnings/__init__.py

@@ -244,6 +244,15 @@ def test_once(self):
                                     42)
             self.assertEqual(len(w), 0)
 
+    def test_once_across_modules(self):
+        with self.module.catch_warnings(record=True) as w:
+            self.module.simplefilter("once", UserWarning)
+            for name in ("module_a", "module_b"):
+                namespace = {"__name__": name, "warnings": self.module}
+                exec("warnings.warn('once across modules')\n"
+                     "warnings.warn('once across modules')", namespace)
+                self.assertEqual(len(w), 1)
+
     def test_filter_module(self):
         MS_WINDOWS = (sys.platform == 'win32')
         with self.module.catch_warnings(record=True) as w:

Misc/NEWS.d/next/Library/2026-10-20-02-00-00.gh-issue-159792.Ym3k6B.rst

@@ -0,0 +1,2 @@
+Fix the ``"once"`` warning filter to suppress repeated warnings across
+modules, matching the documented behavior and the pure-Python implementation.

Python/_warnings.c

@@ -873,11 +873,9 @@ warn_explicit(PyThreadState *tstate, PyObject *category, PyObject *message,
         }
 
         if (_PyUnicode_EqualToASCIIString(action, "once")) {
-            if (registry == NULL || registry == Py_None) {
-                registry = get_once_registry(interp);
-                if (registry == NULL)
-                    goto cleanup;
-            }
+            registry = get_once_registry(interp);
+            if (registry == NULL)
+                goto cleanup;
             /* WarningsState.once_registry[(text, category)] = 1 */
             rc = update_registry(interp, registry, text, category, 0);
         }