Welcome to CF!
Without see a line of code, all we can do is guess.
here are some pitch-black stabs:
1. you are adding an element the first time using .innerHTML that duplicates an element id.
2. the ajax call is caching, and uses the old data to redraw; thus nothing appears to change.
3. there is an issue with the form elements name causing ambiguity in the auto-complete code.
if you could post the part of your code that applies the auto complete from the ajax response, i can likely be a lot more helpful.
or you can just wait until IE8 comes out next year, and hope your problem is one of the 8000 things they have fixed so far...
(2013/5) HTML5:90.2% MOB:15.2% IE7:0.5% IE8:8.4% IE9:8.5% IE10:8.5%