Everlytic has an advanced tool that allows you to merge separate lists into one database.
When you merge two different lists, the old list will automatically be deleted. No duplicates will be created and contacts will only exist in the final list, so there's no need to worry about duplicates or missing contacts. Follow these steps to merge lists:
You can use this option to merge all the separate lists into one master database allowing you to send to one list instead of many.
When you import a contact for the first time, the contact exists in the list you import it to. When you try importing that contact to another list the contact will be associated with that list, because the system picks up that the contact already exists.
So you can send one message to two different lists, where the same contact exists in both lists, and the contact will only get the mail once.