Text (7th+errata): Each player can't cast more than one spell each turn. [Oracle 2009/07/08]
You cannot play a buyback spell more than once. It does not matter that it's the same spell card. It's not the same spell instance. [WotC Rules Team 1998/10/18]
If the spell is countered, it still counts as the one spell played. [D'Angelo 1998/10/19]
When it enters play, it will look back over the turn to see if any spell was cast before it entered play. [DeLaney 1998/10/17] It will notice itself being played. [D'Angelo 1998/10/22]
When it leaves play, the effect ends and all players are able to play spells unrestricted again. [D'Angelo 1999/11/23]
