Yes you can drag and drop contacts between address books. Will it add any duplicates to the target address book?
When all contacts are moved from one address book to the other with the select all and drag and drop function, will it only add contacts that are not already in the target address book, should there be duplicate contacts common to both address books?
When all contacts are moved from one address book to the other with the select all and drag and drop function, will it only add contacts that are not already in the target address book, should there be duplicate contacts common to both address books?
Opaite Mbohovái (1)
Thunderbird will not eliminate duplicate contacts in your address book. That's your responsibility. You may search on AMO for an add-on to do that. https://addons.mozilla.org/thunderbird/