So I just started, and finally earned the gems for the 11 premiums buy
before that, I had cleared up through 6.2

So, that leaves me with 22 cards of 3* or higher, but only 14 different girls...
Do I merge all the duplicates, and fill the rest with 6 two stars?
Do I merge all the 3* duplicates, level the duplicate 4*, and fill the rest with 5 two stars?
Or do I merge 2 duplicates, and level all the rest, ignoring the two stars?

Personally I merge all duplicates since you get an extra inventory slot and skill upg chance. Inventory slots get really powerful once you get to 40+ since mid-grade stuff can have crazy stats. I used 2*s for awhile until i got enough 3*/4* to replace them. Just get the 2* to like lvl 10 until you can replace them.

If your able to I'd recommend farming ?-4 of the current event quests since it gives a lot of gacha seeds and gifts. You should be able to flush out your team with 3*s pretty quick.