this is how people make fake accounts and then spread hate mail to others. if say, for instance the psn user causing the trouble is a jpn account, it may not be looking for the same friend, even if they have the same username.
However, if the account you're having trouble with is from the same country, then this truly may be a mystery. have you tried contacting Sony about this problem? or looked on the internet for any previous incidents and/or solutions?