Rated 5 out of 5 stars
Works well for me.
@Konrad65 @ChrisInVT I needed to open the preferences for the add-on via the menus:
Tools > Add-on Preferences > Remove Duplicate Messages
I then unticked "Size", and then it found duplicate messages.
I believe this is because the size of the duplicates is different to the originals because new headers were inserted into them.