Fix duplicate values in Config.known_args_namespace for append actions

changelog/13484.bugfix.rst

@@ -0,0 +1 @@
+Fixed ``-W`` option values being duplicated in ``Config.known_args_namespace`` when using ``action="append"`` options.

src/_pytest/config/__init__.py

@@ -1533,7 +1533,9 @@ def parse(self, args: list[str], addopts: bool = True) -> None:
         # are going to be loaded.
         self.pluginmanager.consider_env()
 
-        self._parser.parse_known_args(args, namespace=self.known_args_namespace)
+        self.known_args_namespace = self._parser.parse_known_args(
+            args, namespace=copy.copy(self.option)
+        )
 
         self._validate_plugins()
         self._warn_about_skipped_plugins()

testing/test_warnings.py

@@ -1038,3 +1038,12 @@ def test_one():
         result = pytester.runpytest()
         result.assert_outcomes(passed=1, warnings=1)
         assert result.ret == ExitCode.OK
+
+
+def test_pythonwarnings_not_duplicated(pytester: Pytester) -> None:
+    """Regression test for #13484: -W values should not be duplicated in
+    known_args_namespace due to the arg parser being called multiple times."""
+    config = pytester.parseconfig("-W", "error")
+    warnings_list = config.known_args_namespace.pythonwarnings
+    assert warnings_list is not None
+    assert warnings_list == ["error"]