If you have the add-on, there is quite rude, but still working way to easy fix it to display such results.
Open the addon_ipid.php script and modify the following line of code:
if($row=db_simpleSelect(0, $Tp, 'post_id, poster_ip, post_time, topic_id, forum_id', 'poster_id', '=', $userid, $sqlOrd, $makeLim)) {
to that (note I've marked changes in Bold):
if($row=db_simpleSelect(0, $Tp, 'post_id, poster_ip, post_time, topic_id, forum_id', 'poster_id', '=', $userid, $sqlOrd, $makeLim, '', '', '', TRUE, 'poster_ip')) {
The script will group by poster's IP and display only unique IDs then.
There is a little hiss with that hack: pagination will still be displayed for all messages done, but in some cases some extra pages will just not work, so you will have all results on the first page (which is most possible) or split up to the limit which is defined by the modified request.