Duplicate emails can be caused by sync issues or mail server glitches. Try using a dedicated duplicate remover add-on (like for Thunderbird or Outlook), and check your account settings to avoid repeated downloads. Also, make sure filters or rules aren’t accidentally copying messages into folders.

How do I do that? I'm an 84YO troglodyte. Technology is not my friend. I tried to download an add-on fixer for my Thunderbird program--it wouldn't install.