I entwered a code, and it said i got a free bonus entry into the fanta halloween instant win game, bujt had a problem when I tried to enter

it told me that i had received a bonus entry into the fanta hallowen instant win game, and it said i wouod lose the entry if i logged out, so i cliked on the little tab that said play now, and it brought me to another page and it had 3 choices where it said "play with your bonus entry" and it brought me to another page where it expeected me to put a code in, so i put the code i just put in to get the points, and it told me that this code has already ben used, so why is it doing that, and how can this help me once i am off the page i cant just keep it on my computer till i get an answer.