It happens sometimes that you find duplicate mails in your inbox in Thunderbird. This could happen for instance when you pull new emails from the mail server and get disconnected during that process. It could happen then that those emails will be retrieved again when you check for mails. Thunderbird has a nice hidden variable that defines how it should treat duplicate mails and I’m going to tell you how to set it correctly so that duplicate mails are removed automatically.
You have to click on Tools > Options to open the options in Thunderbird. Click on the Advanced tab there and select Config Editor under Advanced Configuration. This looks similar to the about:config shortcut in Firefox then. Enter mail.server.default.dup_action in the filter and the entry should be shown alone in the main window. Now right-click that entry and select modify from the menu. You can change the parameter to the following values:
- 0 – this is the default value. It retrieves duplicate mails like normal mails.
- 1 – if you set the value to 1 duplicate mails will be deleted automatically once retrieved
- 2 – if you set the value to 2 duplicated mails will be moved right into the trash
- 3 – the value 3 finally marks duplicate mails as read
If you want to delete duplicate mails automatically in Thunderbird you select the value 1.

Thunderbird Remove Duplicate Mails
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Thunderbird Duplicate Contact Manager
Thanks for knowing me about config editor.It is helping me a lot.