I'm at a point where almost all of my 3 star units have all their equipment slots unlocked and the ones who can be skilled up with 2 star units are already max skilled, so when I get a copy of a max skilled/max slotted 3 star I've just been feeding it up to level 7 then using it as exp fodder or just selling it. Is it worth keeping more than one copy of a 3 star unit? I'm assuming no, but wanted to double check in case I've been throwing the extras away when I shouldn't be.