Originally Posted by Unregistered

Them of ALL rank ｃｈａｒａｃｔｅｒ unit(2* to 6*) must be combine.
Because those cobined unit added Equipment frame.
This is two Equipment frames that is combined of two units,if there are three Equipment frames that three is.four is same process.

The new units(gacha,drop,etc) are that have only one a Equipment frame.

So later,It is mounted the New Equipment,It is that are four item,ｂracelets, necklaces, rings, but the name was forgotten consisting of another equipment.

Therefore,ｃｈａｒａｃｔｅｒ unit of the Equipment frame desirable more often than is.

The details of which are, I think that wirte a coment of this wiki , blog , youtube and tells us maybe I asked to the Japanese.
There knows.

By that, it would be useful for our play.
More easily, I think more quickly capture can be.

Man... I couldn't understand a thing.

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*