Third, let's assume A1 is the replaced player, A6 is the 'original' sub, and A7 is the 'new' sub. Once A6 is beckoned, A1 is out, but A7 can still replace A6. 
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To make sure the question is answered the 'original' sub can be replaced by a 'new' sub. By the rule the 'original' sub becomes a player and then is replaced before the ball become live again.