I have been freshly re-thinking my problem, and can solve it quite simply:

• Email Marketing DOES filter duplicates, but only WITHIN one Mailing, not within a Campaign. (Maybe Marketing Automation does this also within a Campaign?)
• In the recipient Filter, there is no way to combine recipients from “Event Registrations” and “Mailing Contacts”
• • Importing the registrations as contacts in a Mailing List to workaround this problem does not help, as the Mailings’ recipient Filters take the Mail-ID, not the Email itself to deduplicate the target recipient set.
  • One has to manually label each mailing contact corresponding to the contact registered to earlier Events as a member of a manually created MailingList:“Contacts from earlier Event Registrations”, otherwise duplicate contacts with same email address are not seen as duplicates!!!

e.g. 12 Mailing Lists to be sent in phases:

• Week1: List 1-4
• Week2: List 5-8
• Week3: List 9-12

Deduplication works over ALL contacts in ALL Mailing lists like this:

• Send Mailing1 in Week1 to contacts in List 1-4
• Schedule Mailing2 for Week2 to contacts in List 5-8 AND not in List 1-4
• Schedule Mailing3 for Week3 to contacts in List 9-12 AND not in List 1-8

Basically, you could use odoo studio to make a check on the other lists if a specific email address already exists. The requirement you are expressing is very special for your needs as many companies maintain multiple lists for different purposes with people being part of each list.

In order to invite each person only once, you could create a new list. You export all the lists to this very new list and delete duplicate using excel. Then you can upload the contacts in that new list that will for sure only contain each name once.

Good luck

Eva Pinter