user17689: 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?

user15223: There is no rule. You can repeat an old game as long as your opponent cooperates.
There've been entire repeated games in history, including in the Candidates (Nepo-Nakamura).

user11144: §11.3.1 is the relevant paragraph. It prohibits to "use" any outside information. Clearly, a memorized long theory variant isn't "use" in this sense, the players wanted to play it anyway, regardless that the same thing happens on another board. This is a ticklish situation for the arbiter: where to draw the line?
Well, the FIDE rules also prescribe that an arbiter should have tact and good discernment. Completely lacking former :-), I might politely advise the players to stay the bleep away from the other board until the annoying situation is over, depending on game length.
Note this physical stay-away isn't possible in a team match and even less in a tournament but then, I am an arbiter myself and never encountered this situation. (Note that for foul play, a whopping four players must cooperate, who have pairwise different goals, which makes it a bit improbable to happen.)

17689: <p>I was watching <a href="https://www.youtube.com/watch?v=N-nR8QZUphI" rel="nofollow noreferrer">this</a> 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.</p>
<p>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 &quot;cheating&quot; by the players doing the same move last.</p>
<p>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?</p>
 15223: <p>There is no rule. You can repeat an old game as long as your opponent cooperates.</p>
<p>There've been <a href="https://chess.stackexchange.com/questions/29129/are-there-any-repeated-games-between-gms">entire repeated games</a> in history, including in the Candidates (Nepo-Nakamura).</p>
 11144: <p>§11.3.1 is the relevant paragraph. It prohibits to &quot;use&quot; any outside information. Clearly, a memorized long theory variant isn't &quot;use&quot; in this sense, the players wanted to play it anyway, regardless that the same thing happens on another board. This is a ticklish situation for the arbiter: where to draw the line?</p>
<p>Well, the FIDE rules also prescribe that an arbiter should have tact and good discernment. Completely lacking former :-), I might politely advise the players to stay the bleep away from the other board until the annoying situation is over, depending on game length.</p>
<p>Note this physical stay-away isn't possible in a team match and even less in a tournament but then, I am an arbiter myself and never encountered this situation. (Note that for foul play, a whopping four players must cooperate, who have pairwise different goals, which makes it a bit improbable to happen.)</p>
