It happens sometimes that you find duplicate mails in your inbox in Thunderbird. This can happen for instance when you retrieve new emails from the mail server and get disconnected during that process.
It can happen then that those emails will be retrieved again when you check for mails so that you end up with them being listed twice in the email client.
That's obviously not what you want, and while you can deal with the issue by deleting all duplicate emails manually, it may take some time to do so depending on the number of mails you have received.
Thunderbird has a nice hidden feature that defines how to treat duplicate mails.
I'm going to show you how to set it correctly so that duplicate mails are removed automatically.
You have to click on Tools > Options to open the preferences in Thunderbird. Click on the Advanced tab there and select Config Editor under Advanced Configuration. Note that you may receive a warning message when you open the editor for the first time. You need to accept the warning before you can continue.
If you are using the Firefox web browser you may notice that it looks identical to the one you find in the browser. 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:
If you want to delete duplicate mails automatically in Thunderbird you set the value 1.
You may want to experiment with those options for some time before you select a final value for it. While deleting duplicates may sound like the best option right out of the box, you may want to be sure that no regular mail gets caught in the cross fire. While unlikely, it is better to be safe than sorry. My advise is to use a value of 2 or 3 instead and monitor this for a while before you set the value to 1.Advertisement
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