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Unlike some CRMs, there is no duplicate (=same email address) within Marketo smartlist or list.
So let's say you have created a list with leads that have registered to an event and there is a lead who registered twice in 2 separate batches, they will just show up once. In their activity logs you'll see that they have been added twice though.
In your case, I load the full list. The only time I only load net new or existing is when I have an external list and I want Marketo to show me : how many of the leads within my list are net new or how many of the leads within my list are already existing in my DB