Forums Scripts & Popups Preventing duplicates

Hey guys I'm a little bit stuck.

I've created a simple script that will add the users IRC nick to a text file when they run a certain command detailed below.

on *:TEXT:!giveaway enter:#: {
write giveaway.txt $nick
msg $chan $nick - you have been entered in to the giveaway! }

Is there a simple way to check if the users nick is already in the file and if so not add it again (and maybe inform them that they already entered)?

Thanks!