Then you need to debug mySQL request. IP function uses setup_mysql.php - db_simpleSelect.
Under this function, you need to de-comment:
echo "<!-- ".$sql." -->\n";
then go to the IPs page and view it's source. You will see the list of commands, and there will be one command having DISTINCT statement. Like DISTINCT poster_name, poster_id
... You need to check out this command in the regular mySQL command tool to execute it.
What kind of warning mySQL will give? You will see it's response and could get the idea what's wrong.