Deduplicate pythonwarnings to fix -W duplication

Author: tanuj-rai

src/_pytest/config/__init__.py

@@ -1417,6 +1417,10 @@ def _preparse(self, args: list[str], addopts: bool = True) -> None:
         self.known_args_namespace = self._parser.parse_known_args(
             args, namespace=copy.copy(self.known_args_namespace)
         )
+        
+        # Deduplicate pythonwarnings to fix duplicate -W entries
+        warnings = getattr(self.known_args_namespace, "pythonwarnings", [])
+        self.known_args_namespace.pythonwarnings = list(dict.fromkeys(warnings)
 
         self._validate_plugins()
         self._warn_about_skipped_plugins()