I was looking at playing Megalovania and came across a B flat on measure 41. Normally, I would play a B flat, but since it it is already a B flat from the B flat major chord, I am super confused. Is this a typo from whoever wrote the music, or does something weird happen with the note? I assume it either cancels out or becomes an A, but then wouldn't they just have wrote it as an A or with a natural? How should I play that?
It's called a courtesy accidental. Look at the measure before:
The B-flats are naturalised there. But they have to be flat again in measure 41:
The extra flat in measure 41 is to remind the player that the B's are flat again. So you just play a B-flat there. Successive identical accidentals are never added together, a B-double flat would be depicted with two b's before the same note.