Looks like they (temporarily?) disabled the ability to merge allies with the same star level. I was able to do that a while back when I pulled a second 4* Cherche, and all it got me was essentially a 1 level upgrade (although it shows up as a '+', not an actual level upgrade). Since then, when I've pulled dupes, I've just leveled them up separately or ignored 1 in favor of the other.
Completely unrelated--as I was typing this response, the following popped up:
"This topic is clearly important to you – you've posted more than 29% of the replies here.
Are you sure you're providing adequate time for other people to share their points of view, too?"
Umm ... thanks, Discourse ... ?