If they are duplicates, keep them for limit breaking in the future. Especially valiant unit such as Kane, they are hard to come by.What should i do with duplicate potential 5* heroes with awful faiths(such as [Hera] Zedd or [Athena] Kane). Are they worth levelling all the way to 5*? Or shld they be used as fusion for other heroes.
Once your hero get to 5* +2 max training, they can limit break using the same hero of the same class to increase the max level (+2 lv. each time, max 5 times).