come one, someone gotta know how to merge two lists into one

how can this be done.......

there is a Master and a Secondary list
how do i combine them both into one list called the Merged list box

but if the Master and Secondary list boxes have two items that are the same then only add one of them not both, resulting in, the merged list would contain all the items from both the list boxes, except only add one of the duplicating items.