Depending on the character and how good their abilities are, you can use the extra covers to start a second version of them where you specialize in a different build (assuming you can’t max all the powers in a single build). Otherwise selling them is really the only other alternative currently.
You’d be better off selling it and making room for new characters as it seems like a waste of ISO to raise two of the same characters, especially since you can only use one at a time (or neither of them in certain tournaments). I say sell it and save your iso on another character