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Hum, actually this fix is not enough. When verbose_feature_names_out=False, it doesn't use get_feature_names_out but keep the current column names instead (i.e. with the meaningless intermediate names that I added here).

So a proper fix would be to do something like in the ColumnTransformer, that is if verbose_feature_names_out=False, obtain each transformer out feature names and use those before stacking, and raise an informative error in case of duplicate column names.