This is very normal - there’s an index for each way of accessing the table. It’ll slightly hurt your write performance and on-disk size, but not enough that I would worry about it in any way; generally, unless you know you’re going to be write-constrained (which is rare and limited to things like rapid trading), the more indexes the better.
Specifically, here it looks like every foreign key has its own implicit index in a many to many join table, along with one for the (a, b) column pair. I’d say this is quite normal and actually implicit in the FOREIGN KEY statement if I remember right; I’m not even sure how we could easily turn this off.