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CloseReview for Remove Duplicate Messages by naasking
Rated 5 out of 5 stars
It works great except on "special folders", like sent mail. In migrating email to another host some duplicates happened and this add-on would have been perfect to clean this up, if not for this one limitation.
Edit: the author notified me of a setting to toggle this behaviour, so no complaints now!
The extension _does_ work on special folders - if you ask it to, in the options dialog. By default, it skips them, because usually people don't want to check for duplicates with folders like Drafts or Archive where one often has copies of the same messages.
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