Fix: Prioritizing False if self.flags.allows_duplicate_labels or other.flags.allows_duplicate_labels is False

Author: fbourgey

Diff for: pandas/core/generic.py

@@ -6076,8 +6076,10 @@ def __finalize__(self, other, method: str | None = None, **kwargs) -> Self:
                 # One could make the deepcopy unconditionally, but a deepcopy
                 # of an empty dict is 50x more expensive than the empty check.
                 self.attrs = deepcopy(other.attrs)
-
-            self.flags.allows_duplicate_labels = other.flags.allows_duplicate_labels
+            self.flags.allows_duplicate_labels = (
+                self.flags.allows_duplicate_labels
+                and other.flags.allows_duplicate_labels
+            )
             # For subclasses using _metadata.
             for name in set(self._metadata) & set(other._metadata):
                 assert isinstance(name, str)