To begin I've dated this girl before and it didn't end on bad terms. But the problem is that after me and her broke up my friend started to date this girl months after we ended unknowingly that I have already dated her. Not to mention me and this girl are not only each other best friends but also each other first loves too. I'm number one in her head always and she is by number one too but they also had there moments together too. So my question is should I go back with this girl after she has been with a person who I call friend but remember I did state that we were each other first love. I have already made my decision on the matter but to hear thing from other people point of view would help a lot.

Well it sounds like you have already made your mind up but I think if you were any kind of friend to this other person then I would talk to them first because if you don't you could end up hurting your friend and end up losing them

If you and she really care about each other, then your friend will understand. If the friend can't accept it, then he isn't much of a friend. Like berta146 said, sit down and talk it over... that is good advise.