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...
BROWSER STATS [% share]
(2013/10/31) IE7:0.5, IE8:8.6, IE9:5.3, IE10:12.3, FF:17.7, CH:41.8, SF:8.1, MOBILE:20.4