At the start of the movie when the muskateers are disbanded, they are all required to burn their muskateer cloaks and leave their swords...but closer to the end of the movie, when the muskateers join together again, they all pull their cloaks out of secret hiding places along with their swords!
I don't know I think that they could have an extra pair but then again I knida think that they don't and it is strange that they just appear. My dad thinks they have an extra pair of outfits.
This is not a slip-up. All of the Musketeers were loyal to the king, they could see that there was something wrong. They only burned the cloaks they were wearing. Do you only have one work shirt? Obviously they had more than one, and just hid their cloaks and swords for a time to rebel. And a good thing they did...