It depends on the BB ability and the situation. Having two Mitigation units for example can be extremely useful for a lot of content even if they are the same. However unless you are F2P and don't have a lot of options for high hit count spam teams and/or not a lot of Earth Units for the eventual Earth Vortex/Arena, I wouldn't keep multiple Miku.
The one advantage to multiples of a unit is very easy sparking, which is really useful with certain builds.
Multiple Miku would be useful for a high hit count BB spam team if you're F2P - 27 hits on her BB is pretty good, but the buff only is applied once, so you'd lose out on that. It's still pretty good when doing Metal vortexes, though.
I have 4 Zelnites, so I'm eventually planning on making a team of 4 Zelnites + Elza and pop Elza then all 4 Zelnites and watch the spark damage overwhelm, but typically it's not something you can do unless you pulled too much
I also have 5 Micheles, which I really don't need (hell even one is kinda useless now) so I won't keep all 5 (I also have 4 Grybes, which are all useless) but Miku I would probably keep, actually, especially if you're F2P and you don't have access to other high hit count BBs. If you have better units, though, then yeah, keep one (preferably an Anima one) and get rid of all the others (use them to level your BB from 7 to 10, which would be their perfect usage).
depends on the unit. maxwell is quite a bit easier with 2 rowgens/lilith for example.
God, a team with Zelnite lead + Ulkina + Darvanshel + 2 Rowgen + friend Grahdens would most likely destroy Maxwell quite easily, I can imagine... That's a *lot* of damage and if you can spark them off each other you've pretty much won and you generate a lot of BC (Rowgen has 30 drop checks on his SBB, which is the same as Elza) so you should be good. I can actually build that team... Hmmm..
I think I'll still go with the safer route and go lead Lilly + Darvanshel + Ulkina + Rowgen + Zelnite + friend Grah. Lilly will boost up my BC quite a bit more, Zelnite for BB-gauge manipulation in tough spots, Rowgen for super damage... Should work well. All the members save Lilly that I got this morning are all SBB10, so it would most likely be doable.