From the Super Mario Wiki
Bowser and...Bowser? is a post-game level in the Bowser Jr.'s Journey mode of Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey.
After hearing about Bowser Jr.'s adventure, Bowser invites him for a duel. However, the two end up fighting so much that the fights get very chaotic and the castle gets badly damaged. After the level is completed, Bowser joins the team.