Forums Scripts & Popups /write -s

How can i get this function to work:

/write -s"$nick $1" blah.txt $nick $1-

So itll read blah.txt to see if its already there, if so, it overwrites that line, so there are no duplicates.

Thanks in advance.

//write $+(-s",$nick $1") blah.txt $nick $1-