You would have to use materials to level up both of your duplicates.
They'll ending up getting merged together eventually, so I would count it as an opportunity loss of those materials.
You always wanna prioritize leveling characters you think you'll keep around the longest.
You always wanna look at the event running that week and what character will be replaced when you receive them.
Decide which of your team members is "on death row" and try to hold off on using stuff to level them if possible.
You'll reach a point in the game when you are scrounging to level your good characters and you'll be thinking to yourself "why did I waste all those good red manyus leveling that duplicate character?!"

I combine even my 5* girls dupe girls (right 3 equip slots Orchid? ¬¬). At some point you will even ditch 4* girls to replace them with 5* event girls, so there is no point dedicating much rerources to duplicates, just level up them enough that you can clear the stages, opening equip slots also helps you to increase their power without speding resources on them.

Pretty much what other people have said. Merge the 4* to make them more useful as eventually you will phase them out from the squad for event 5*