I did the same thing....I created my first account with an email address (no facebook attached). Afterwards, I tried logging in with my facebook account and it created a new account for me; even though my first account matched the email of my facebook.
I never found out how to link the two, so what I did was:
1) Go onto my first account and change the email address to a different one.
2) Go onto my facebook nihongo account and associate the email to the original one used.