When a new lead from Swift Digital arrives and an existing lead already exists in Salesforce, I end up with duplicate leads what should I do?

Salesforce allows duplicate leads, as such in order to merge duplicates. Use 'Duplicate Manager', a Salesforce native function, to merge incoming leads to existing leads. Ensuring the incoming lead is the winning leader of this merge. Duplicate manager is best used for any incoming leads.

https://help.salesforce.com/articleView?id=managing_duplicates_overview.htm&type=0

Mass de-duplication can also be performed. There are many Salesforce APPs in the marketplace that can be use for such functions.

For mass conversion to Contacts use APPS such as this:

https://appexchange.salesforce.com/listingDetail?listingId=a0N300000030ptZEAQ

If you do not wish to create new lead in Salesforce turn off this function in Salesforce Configuration page.

Note:

When you add members to a shared mailgroups in SF, new contacts will be created in SD, and if there is an existing match SD will do an auto merge and provide a Swift person ID, linking the 2 together. This by far the easiest way to connect members form SF to SD.