DataSet subtraction and distinction

Data processing tools, can remove the duplicate data in the data set. Can deal the two sets of subtraction. The use of the scene: For example, I am a network seller, want to send to many customers of information in order to promote new products; Sent to only the online users, and the users had already been sent yesterday do not need to send again. This requires the user has been sent from the current list of online users to be removed. And the list of data need to be distinct, so as not to send multiple times to a user.