Is it OK to do a grad school interview if I was already accepted informally by another program?
I would advise that, until you have a formal, firm offer letter, you keep following the steps of the rest of applications you submitted. Even after making you an admission offer, you will have some time to consider any other option and decide (many people get accepted in multiple programs and it is fully alright and understood). Now, once you accept an offer (not only receive it), it is very recommended that you contact the other programs that have made you an offer, and tell them that you will not be attending there because you already accepted an offer somewhere else. They will all understand, of course. But, most important, if you tell them as soon as you have decided, then they will have some time to make an offer to another candidate.