How do I transfer my Thunderbird address book to a different computer running Thunderbird, and combine the old and new list into one?

I have a computer at my summer residence and one at my winter residence. They both run Thunderbird for my emails. When I move from one residence to the other I wish to update my Thunderbird address book. I do this by selecting - Address book - tools - export, at the "old" computer, to store them on a memory stick, and _ Address book - tools - import, at the new address to load the list from the memory stick. It works, except the old and new address lists are separate lists. I would like to combine them, but without duplication of the same addresses. Is this possible? How would I do this?