There are two reasons Full Tilt might think you have a second account when you don't:
(1) Someone with the same legal name as you also has an account.
(2) Someone in your residence also has a Full Tilt account--especially if you both play on the same computer.
If you don't have a second account, send an email to
support@fulltilt.co.uk explaining the situation. They will probably ask for copies for government ID with photo and signature and a document showing your registered address. Be aware that Full Tilt support sometimes takes a little bit of time to respond to emails. If they're too slow for your liking, create a support ticket here on PokerStrategy and ask to have the Full Tilt key account manager look into it.
If everything is straightened out, you should get your free $50, but there is no guarantee that Full Tilt will accept your explanation. Either way, it has to do with the poker site and there is a limited amount that PS can do.