validation - Javascript, check for identical characters -

i got make script checking input box (password) same characters occuring twice. should used alongside regex validation (that's working).
succeed know need use (for?) loop somehow, checks if 1 character appear twice. kind of odd thing ask for. know. i'm not entirely sure conditions function. if have suggestions around how made, great. example: "abad12" - pass, whilst "abac12" return false. in advance.

function checkform(form)   {   var re = /^\w{6,10}$/;    if(!re.test(form.pwd1.value)) {   alert("error: password has in-between 6-10 characters!");   form.inputfield.focus();   return false;   }   } 

above goes example of script i'd combine (among more regex validations).

that's how this:

var str = "abad12",     valid = str.split("").filter(function(e, i, a) {         return a.indexof(e) !== i;     }).length === 0;  if (!valid) {     // ... } 

but note array filter() method not available in old browsers, check browser compatibility page in advance.