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*