I get the same thing and no one has figured out what it means. I did email the IPO and they told me that when we check in we should ask about the BOG FastPass+ and they would see what the can do.
I had this exact same thing when we went in August! I asked at the resort concierge desk about getting a FP+ reservation. He handed me an iPad and told me to log into my account. Of course, I knew what message I'd get and I did. He was completely puzzled as well. He did give me a BOG FP ticket with my reservation number written on it and told me to go to guest services at MK.
So on MK day, I head to guest services. They hand me an iPad and tell me to log into my account. Guess what message I get yet again! She has no clue either. After about 10 minutes of her going in back and asking someone else, trying to figure it out, etc., she comes out and goes to someone else with a different iPad (a CM who actually is dealing with BOG). They look at it and can't figure it out either.
But they do ask me what time I'd like the FP and hand me another one of the same BOG FP tickets with my name, party size, and FP time written on it, along with her name. She says she'll call ahead and let them know. At the appointed time, I went to the check-in desk at BOG. They had no clue. But I explain it to them, show them my (now) 2 FP tickets, and they send me right up the FP line. I get to go right in, order, and have a lovely lunch!
So, long story short, no one seems to know what that message means, but it doesn't necessarily lock you out of the FP BOG. Just might take some work!
I had cleared out cookies on my computer, used every reservation # I could find, logged in with a different account, changed my password. You name it, nothing cleared that "guest is already registered" message. But by spending a few minutes at guest services, I got to avoid standing in line, in the hot sun, at the restaurant.