diff --git a/src/_pytest/config/__init__.py b/src/_pytest/config/__init__.py
index 38fb1ee6d27..fa116d732ac 100644
--- a/src/_pytest/config/__init__.py
+++ b/src/_pytest/config/__init__.py
@@ -1418,6 +1418,10 @@ def _preparse(self, args: list[str], addopts: bool = True) -> None:
             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()