Second Signature Code [RD]

Currently on my site I am using this code:

<script type="text/Javascript">
// Second Admin Editable Only Signature by Chris
// Do not redistribute without permission of the creator

// Scroll down til it says "EDIT HERE"
var aTD = document.getElementsByTagName("td");
function addSig(user,sig){
   for(a=0;a<aTD.length;a++){
       if(aTD[a].width == "20%" && aTD[a].innerHTML.match("user="+user)){
           if(location.href.match(/action=viewprofile/i))
               return false;
           var x = aTD[a].nextSibling.getElementsByTagName("table")[0];
           var y = x.rows[2].cells[0];
           if(!y.innerHTML.match(/<hr/i))
               y.innerHTML += "<hr size='1' class='hr' />";
           else
               y.innerHTML += "<br />";
           y.innerHTML += "<font size='1'>"+sig+"</font>";
       }
   }
}

// EDIT HERE
// SYNTAX: addSig("username","sig's HTML. No line breaks, use <br /> instead");
addSig("admin","Whee.");
</script>

but there is an issue: if someone has a similar username, they get the same signature. Like, one member is using the name "Ren" and another user is using the name "Renei" and though I haven't modded in Renei's signature, they are recieving the signature that I have given Ren. One of my mods name is "Mav" and one of their character's names is "Maverick", so one of the accounts got two signatures. It also happens when people use numbers wit their multiple accounts. . . . "guest1" "guest2" etc etc.

Is there some way for me to fix that?