"The situation where the penalty taker accidentally kicks the ball with both feet simultaneously or when the ball touches the penalty taker's non-kicking foot or leg immediately after they have taken the kick... is rare," wrote Lukas Brud, the secretary  of the International Football Association Board (IFAB) in a circular.

It did occur in the Champions League round-of-16 second matchup between Atletico and Real Madrid in March.

The tie finished 2-2 on aggregate and went to a shoot-out.

On his attempt, Alvarez slipped but still found the net. Video review detected that he touched the ball twice as he shot. The referee ruled the shot a miss under Law 14, which deals with the penalty kick, and Real went on to win 4-2.

After the match, European governing body UEFA said that "under the current rule, the VAR had to call the referee signalling that the goal should be disallowed".

UEFA said it would hold talks with FIFA and IFAB.

Brud said the situation had not been addressed by the existing rule.

"As it is not directly covered in Law 14, referees have understandably tended to penalise the kicker for having touched the ball again," wrote Brud.

However, he added, the law "is primarily intended for situations where the penalty taker deliberately touches the ball a second time before it has touched another player".

"This is very different from the penalty taker accidentally kicking the ball with both feet simultaneously or touching the ball with their non-kicking foot or leg immediately after they have taken the kick, which usually occurs because they have slipped."

Brud pointed out that even an accidental second touch could be unfair to a goalkeeper because it changes the ball's trajectory.

Therefore, he wrote, IFAB had decided that "if the kick is successful, it is retaken".

If a kick during the game is unsuccessful, the result is an indirect free kick, as it would be for a deliberate second touch, unless the referee decides to play an advantage for the defending team. In a shootout it remains a miss.