I use K-9 Mail on my Android phone as my email client. It’s ugly, inelegant, and quirky, but it works, and has all the functionality I need.
One of its quirks is that if, under Account Settings > Folders > Sent folder, the folder is set to “Sent”, I end up with duplicate copies of every email I send in that folder. I’ve been using K-9 long enough that I’ve ended up with thousands of duplicates like this.
Fortunately Fastmail, my email service provider, has an easy way to address this:
This is a utilitarian feature that allows duplicates in a selected folder to be eliminated. I did this yesterday, and it worked like a charm.