We got duplicates too...planning to take the extra set on our trip to get sorted out on arrival. Like others, it shows I'm managing 12 bands (all active), but I can only get FP+ for 6 of them.
However, I think I received duplicates because the "avatars" associated with my 2 separate reservations (4 immediate family / 2 in-laws) were initially messed up with 2 of the six "avatars" not being linked to either reservation and 2 of the other six linked to both reservations. When I customized my MBs, 2 of the avatars showed up with 2 bands (and 2 with none), so when I went to do FP+, I could only get 4, not 6. My MBs were already being processed, so I think that's when the first shipment was mailed. After calling Disney, they fixed the reservation/avatar issue and I was able to re-customize (this is before I got the first shipment of MBs but probably after they were already mailed to me), and Disney must have processed a new MB shipment for me as the first batch was probably already out the door on their way to me.