If you need a unique list based on one you have, you just input that one like so:
If you have to different lists or a list and a object you could union those and get only unique objects.
It's possible for your list to contain duplicate objects. To get a list of unique objects afterwards use the list operation ‘union’.
If your list iassociated with another table 1->* then it will only include unigue items, if you add it twice it will be there once.