Any recommendation on an effective way to stop duplicate leads ( i am not really a sql junkie

What would be proper sql query which will select phone number of leads with status as sale and check database for other entry with same number and mark is as SALE too ?
/EDIT: Got it sorted .. copied all sale number in another table ( once if they exists multiple time ) and then update vicidial_list table comparing from other table ..
I think there must be option in lead loader to check for dupes from all list assigned to single campaign .