Are you importing lead with a CSV file? Marketo will dedupe automatically by email address.
If you are adding leads to an existing static list (in any way), Marketo will just add new leads to the list. if the lead is already a member, it will be ok. It will not overwrite the leads already in the list or push them out. It will just add the leads who are not already members.
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
Thank you, Josh and Cecile. I appreciate your quick answers.
I am uploading a .csv file. Moving forward I will plan to update my .csv file and add to my existing list.