Foul play

So yesterday i bought a box of marshmallow Easter eggs for my family. noticed that there was a golden egg competition running, checked all my eggs for the stickers and fount that there were seven with those competition stickers, i played six times for a chance to win a family holiday, followed the instructions -perfectly. my seventh egg was a golden egg. excited i played -guess what i got a message saying someone had already used that code, now can someone tell me how that is because i opened that ester egg, i unstuck that code from the wrapper. you guys if you are not willing to let people win these competitions why do you ryn them in the first place? am i being cheated here. can someone explain