Here is FAQ costume: Hero Costume FAQs! Please read for answers to most questions before posting new questions
Costume only need 1x, dups only valuable as feeder for Hero-Costume-Yellow.
So, you can choose which Bane you want to max, and which maxxed Bane want to level his costum.
Bane 1: without costume maxxed, costume maxxed
Bane 2: without costume at level 1.1, costume at level 1.1
Bane 3: … and so on.
Each Hero have seperate level both (original and the costume).