move data in between two listboxes without duplicate

Question

Haii i want move data between two listboxes by  using buttons like(<,>,<<,>>)  i want move without duplicate data for  example,in listbox1 have some countries (America,greenland,africa,srilanka...) and listbox2  contains some countries like(germany,pakisthan,nepal,bangladesh,america,srilanka...etc) when i selet listbox1 item for move to listbox2 ,it will compare and move if it is not in listbox2
suppose i select america,it is already in listbox2 so the message will display it already i listbox 2
otherwise it will move listbox2.