Who, Skuld? Keep her! Hard to come across 6* GFEs. You also can't undo sells. Certain teams may just fall into your box over time.
My Sumire team appeared over the course of about half a year.
My ABastet team appeared over the course of a year, with the final piece--Michael--appearing just this year. I used Athena as a stopgap, though, so you could argue that ABastet, who I got last PCGF, was the final piece xD and there are still a lot of better options that I could use that I don't have, like Verdandi, Vishnu, Liuuu Bei...
I finally found a use for my second Claire over half a year after I got her, and I never foresaw that I would want to use that second Claire on an AYomi team, despite how long AYomi's been out (I only finally got AYomi a little while ago, and I only started using an actual AYomi team even more recently!) sooo if I purged that second Claire just because I couldn't see a use for her... man would I cry hard
Dark Dragon Knight is finally seeing regular use after over half a year--and I never foresaw that he would become a half-staple of my AYomi team
I got my first DQXQ in June 2014 and my second DQXQ in May 2015. I never used my second DQXQ (who I turned into DQ) regularly. When ADQXQ hits NA, though, I will switch my second DQXQ to ADQXQ, because I very much like my XQ (my first DQXQ), and I may end up playing them both regularly.
I pulled two Hades in a row in my early PAD career. I went on to max the skill of one of them the long way (via Heras). The other one is sitting in my box underleveled because he has 1337 HP, but I could easily turn him into Awoken Hades if I wanted to without losing my handy little DL Hades with his God Killer and 15-turn 30% gravity.
I have three LKalis. I only use one of them regularly. When Skill Inheritance was announced, I started to develop the second one for Skill Inheritance (or whatever NA will call it). I plan on developing the third for Skill Inheritance as well.
My Sumire team often has two Skuld actives; one from me, one from a friend (Skuld/Sumire will be even more fun once Skuld's leader skill is updated haha)
I regularly run my Ronia/Lu Bu team with three Ronias, including a friend's Ronia, although I sometimes prefer to run it with four Ronias (yeah I have three Ronias in my box). A special case, but still.
If you are only a little bit of MP away from something that you want to buy from the MP shop, then that's a different matter. If you're still pretty far away and/or you don't know what, exactly, you want to get from the MP shop, then hang on to monsters that you don't know what to do with, even dupes.
pew pew
