Let me provide more info. We also have a button (we call it 'search button' below) to trigger the postback. in the event handler of that button. These 2 lists are to be used as one of the conditions for DB query.
When the page is initially loaded, e.g. list rlbUnselectedStataus has 8 different items. And rlbSlectedStatus is empty. We double-click 2 items in unselected list and these 2 items will be transferred to selectedlist. Then we click the search button to get some result. Until this step. everything is ok. But when we click one item in the selected list, now the total number of items in selected list should be 1. When I click the search button again, the items in unselected list is now 3 items (2 duplicate items for the current-still-in-selected-list and one for the just-transferred). So the query result is still same with the previous one.