What does "Use aggregation" option under Other in the Views UI do? It looks like it might be a function that can reduce duplicates rather than using the Distinct option.
Technical what it does it allows you to add "GROUP BY field" and "count(field)" to the sql query. This might be able to reduce duplicates but in general if you have duplicates it's most often a problem by design/your choosen view type(node, user, terms etc.)