I use Thunderbird as my email client. I've used it ever since migrating from Netscape.
A few days ago, I decided to tidy up my Inbox, do some archiving, etc. I noticed a lot of duplicate messages. I spent a while manually deleting them, but eventually realized that was insane. I found an "extension" for T'bird that automatically removes dupes. It removed over 15000! Somehow since then ANOTHER 1700 have appeared. It's not a huge hassle to just push the button and have the system remove them, but I'd like to know what's causing them and fix it (if it's an easy fix).