Talking about accepted offer in job interview
If you accept an offer you should immediately withdraw from consideration from all other potential employers. This is simply to preserve your own credibility for the future. Failing to do this will, perhaps, cause them to expend resources wastefully and it can come back on you in the future. 

They won't want to talk to you in the future if they think you might do this again in the middle of negotiations. 

Alternatively, you can back out of your earlier acceptance if possible so that you are really available. This is risky, of course. But this is a situation that if you try to play games, you will probably lose.