There is an addon for this, but years out of date. Here are some steps that may work for you. - import the new addresses - now, highlight the merged addressbook and export to csv format - load into a spreadsheet and use the spreadsheet facilities to sort (all rows except first row) and remove duplicates - save as csv - start thunderbird and remove all addresses - import the csv

The downside is that some miscellaneous data may be lost that was in one of the duplicates. My suggestion is to just do the merge and, over time, work through the duplicates. I suggest this because any automated approach may cause phone numbers, street addresses, etc., to be lost.