Check out the list actions https://docs.mendix.com/refguide/list-operation
Union is the one you're looking for I think, just pass the retrieved list in the list operation UNION and you get an unique list.
Do you mean unique objects? How do you retrieve duplicate objects by association?
There should never be duplicated in a list that has been retrieved by association.
Unless of course you meant that there is some attribute eg. ‘Name’ in your retrieved entity which might be the same for multiple objects?