I was watching this Firouzja interview by Belenkaya after the last round of the candidates and she asks him if he noticed the similarity between his game and Rapport-Radjabov at the opening.
Despite rarely even imaginable to happen, this makes me wonder how completely identical positions at an advanced part of the game (say, around move 20) in two boards at a tournament round would be handled. I know that there are very theoretical lines which could go beyond move 30 that GMs know by heart, so it wouldn't be automatically "cheating" by the players doing the same move last.
Is there any prescribed rule for such cases? What is the case where this has happened with the largest number of moves played in history?