I'm really sorry to hear you're having an issue with your registered devices.
I cant explain why this has happened for you i'm afraid, If you are still having difficulty or reached your device limit we can always reset that from here for you so you can register both devices again.
I've just had the same issue. I can't watch Virgin TV go as "I've registered two other devices". This is plainly not true. A look at my devices shows two entries for the same phone I'm now trying to watch on. There is clearly a bug here.
Thanks but that didn't help. I still have the same issue that I can't remove the "two" devices that I have allegedly already used, until 1/7. These both show as Sony G3121 and relate to the only device that's ever been used for Virgin Go on my account. So it seems that Virgin Go thinks I'm trying to login on a 3rd device, coincidentally also called Sony G3121.