The tournament is single-elimination. Allowing players to have two chances at a prize while others cannot do the same is an unfair advantage. Sadly, players who like to use both iDevice and PC still have to abide by the rule in order to maintain a level playing field.

IP addresses can be noted and used to identify someone doing this, but further inspection must take place in order to account for two people on separate devices simply in the same household. In this way, siblings playing in different tournaments is suspicious, but legal.

This is a rule because it disallows abuse of the system. Abuse still can occur in different ways (and it does, quite often..) but ideally, everyone should follow the rule. Anyone who doesn't should be reported to a GP as the tournament should be kept fair.