do cards stack?

I have two dusk crusader cards. Do they stack?

Nope…you actually weren’t supposed to be able to receive multiple copies of the same card, but there was a bug that only got fixed recently that allowed you to find the same one repeatedly.

do I have to worry about the two in my inventory or is it just a neat bonus?

Just a neat bonus - there won’t be any negative side effects, or bugs or crashes or anything like that if you have 2+.