In 2004 the British illusionist, Derren Brown, despite being a very poor chess player scored +4 -3 =2 against 9 strong chess players including scoring 2/4 against 4 grandmasters in a simul. Of course it was a blind simul (the players couldn't see each other) with alternating colours where he just copied moves between pairs of boards.
All good, clean fun when performed as entertainment, but what happens when it happens in real life? In a serious competition? What are the sanctions? What can the arbiter do about it? Is it even illegal?
When this was put to Geurt Gijssen in An Arbiter's Notebook his reply was basically nothing!
In my opinion, an arbiter has no possibilities to forbid this behaviour.
Stewart Reuben describes a situation which occurred in an U13 match between England and Sweden about 30 years ago where the Swedish players with white copied the moves made by the English players on the boards next to them and similarly the Swedish black players.
Once the English players realized what was going on one of them, Darren Lee, deliberately lost a piece to see what would happen. The blunder was faithfully copied by the young Swedish player. Eventually the English boys solved the problem by just sitting there until they were in acute time trouble and then blitzed their opponents who didn't have time to continue with their cheating and England ran out 3-1 winners.
Stewart Reuben's suggestion is to make use of article 11.3 a.
During play the players are forbidden to make use of any notes, sources of information or advice, or analyse any game on another chessboard.
However it is a stretch to describe what is going on as "analysis on another chessboard" and it is unclear if you can actually impose a penalty based on this.
The only solution he could suggest was moving same coloured boards affected to another room or another part of the same room to prevent the copying.
In case you think this sort of thing doesn't happen at the highest level here are 3 games from the round 13 of the 1955 Gothenburg Interzonal, although I suspect no way were any of the black's copying!
[White "Keres "]
[Black "Najdorf, Gothenburg 1955 round 13"]
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w - - 0 1"]
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nd4 Nf6 5. Nc3 a6 6. Bg5 e6 7. f4 Be7 8. Qf3 h6 9. Bh4 g5 10. fxg5 Nfd7 11. Ne6 fxe6 12. Qh5 Kf8 13. Bb5 Kg7 14. O-O Ne5 15. Bg3 Ng6 16. gxh6 Rh6 17. Rf7 Kf7 18. Qh6 axb5 19. Rf1 Ke8 20. Qg6 Kd7 21. Rf7 Nc6 22. Nd5 Ra2 23. h4 Qh8 24. Ne7 Ne7 25. Qg5
and
[White "Geller "]
[Black "Panno, Gothenburg 1955 round 13"]
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w - - 0 1"]
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nd4 Nf6 5. Nc3 a6 6. Bg5 e6 7. f4 Be7 8. Qf3 h6 9. Bh4 g5 10. fxg5 Nfd7 11. Ne6 fxe6 12. Qh5 Kf8 13. Bb5 Ne5 14. Bg3 Bg5 15. O-O Ke7 16. Be5 Qb6 17. Kh1 dxe5 18. Qf7 Kd6 19. Rad1 Qd4 20. Rd4 exd4 21. e5 Kc5 22. Qc7 Nc6 23. Bc6
and
[White "Spassky "]
[Black "Pilnik, Gothenburg 1955 round 13"]
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w - - 0 1"]
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nd4 Nf6 5. Nc3 a6 6. Bg5 e6 7. f4 Be7 8. Qf3 h6 9. Bh4 g5 10. fxg5 Nfd7 11. Ne6 fxe6 12. Qh5 Kf8 13. Bb5 Kg7 14. O-O Ne5 15. Bg3 Ng6 16. gxh6 Rh6 17. Rf7 Kf7 18. Qh6 axb5 19. Rf1 Ke8 20. Qg6 Kd7 21. Rf7 Nc6 22. Nd5 Ra2 23. h3 Qh8 24. Ne7 Ne7 25. Qg5 Ra1 26. Kh2 Qd8 27. Qb5 Kc7 28. Qc5 Kb8 29. Bd6 Ka8 30. Be7 Ra5 31. Qb4
I once did something similar myself. My opponent, white, outrated me by about 200 points but turned up about 30 minutes late. meanwhile I kibitzed the game of a player about 400 points stronger than me who often played one of my favourite openings as black. He got a powerful attack although eventually his opponent neutralized it and then my opponent turned up and I had to concentrate on my game.
I followed the moves I'd seen played on the higher board and was delighted when my opponent unwittingly did the same and I got the same powerful looking attack. At a key moment we ended up repeating moves when my opponent alternately checked me and then defended with the checking piece while I alternately blocked the check and then moved the blocker back into the attack, but still a good result.