Have Thunderbird on old desktop, email history going back over a decade. Began using laptop, setup Thunderbird and using it as main email. Was using mailwasher to scan headers, delete unwanted, leaving the rest on server. Every week or so would fire up desktop and D/L emails. Over time grew lazy and stopped this practice. Now want to set up new computer w/Thunderbird. I know how to migrate emails from one computer to another, done it several times. What I need to be able to do is merge the emails from the laptop with the transferred emails from the old desktop. If this results in duplicate messages, I can deal with that.