HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

A question arose regarding the F2 function, specifically when the "Multi-instance detected" message appears. In what situations could this trigger? The person I was playing with, who received this message, says that nothing else was running besides the in-game chat (the chat itself doesn't affect it, I've checked). The player is well-known, and others vouch for him, so there's little doubt about his integrity – what else could have caused this message?
P.S., he wasn't playing another game simultaneously, according to him.