If you already have an account for whatever reason, MAKE SURE YOU TELL THEM!!!! Getting a second account is a show stopper.
I had an NMCI account when I arrived in P'cola, courtesy of my summer stash job at Navy Region Northeast. I told them about it. They ignored that fact, or, more likely, had some random ensign who didn't know what to do with that information dealing with the list, and gave me a new account. Once I figured out that I was no longer first.last and now first.m.last (they never bothered to tell me), I had to get my CAC card changed. Then changed again because PSD messed it up the first time. Then I had to call NMCI. In my experience, no phone call to NMCI lasts less than 45 minutes. I thought it was all sorted out forever...until I got to my new command.
There's still no first.last out there, so the RAG signed me up for all the email listserves with first.last, not first.m.last. It's taking me several months to get NMCI and the RAG's ITs to get the correct email address on the correct listserves. I don't see this issue disappearing, ever.
Moral of the story - don't just tell them you already have an account. Harass Flight Management about it until the LTs in charge AND the senior ensigns who've been working there for a year and a half AND the junior ensign who has been tasked with data entry for NMCI accounts ALL know your name and that you have an account, and that they shouldn't replace it. Then make sure your account doesn't fall dormant.