Forums Scripts & Popups isin .txt help

i'm trying to do a if(%id !isin $read(id.txt))

if (%id !isin $read(id.txt)) {
var %file = scripts/blah/id.txt
var %a = write %file
%a %id
}

%id is part of a socket that takes id:s from urls.
so if the %id isin id.txt i don't want it to write it again.
id.txt:
1fb26ee
e84086
etc etc


my brain is gone.