The below usage seams to work fine in my tests.
Query:
Code:
SELECT `id` FROM `#__comprofiler` WHERE `id` = '[value]' AND `id` != '[user_id]'
Validate On: Successful Results
Success Message: User exists.
Error Message: User does not exist.
Ajax Validation: Enabled
Ensure CB Query Field and CB it self are both up to date then try the above.