If it's mixed onto a single track, no. You're stuck as-is. Once it's mixed, you can't unmix. In theory, it might be possible to write an algorithm that expects an EXACT copy to reappear an EXACT number of samples later, and subtract that. But I don't know of any readily-available tools that have that function, so you'd have to write it yourself or have someone do it for you. If the two copies are different, like one from a direct-wired input and the other from a mic in the room, all bets are off.
If they're on different tracks, not mixed yet, then you can just turn one off. I suspect it's the former though, or you probably wouldn't be asking.
