The keyword UNIQUE means that only last post is kept in event queue for each user and for each event. For example after POST EVENT EV(1) and POST EVENT EV(2) only EV(2) is in event queue if EV(1) is not processed before EV(2) is posted. Event EV(1) is discarded.

The keyword DATA UNIQUE means that also event parameters must be unique. So after calls POST EVENT EV(1), POST EVENT EV(2) and POST EVENT EV(2) events EV(1) and EV(2) are kept in event queue. First EV(2) is discarded.