Hey,
I recently ran into the same issue where Thunderbird sending duplicate emails or saving two copies of every email in the Sent folder. In my case, it turned out that both Thunderbird and the email server (such as Gmail or another IMAP service) were saving sent messages, which resulted in duplicates. This can also happen because of incorrect account settings, synchronization problems, multiple SMTP configurations, antivirus interference, or add-ons that affect the sending process.
To fix it, first check Account Settings > Copies & Folders in Thunderbird and make sure only one option is configured to save sent messages. If you're using an IMAP account, verify whether your email provider is already storing sent emails automatically. You can also repair the Sent folder, compact folders, disable unnecessary add-ons, and update Thunderbird to the latest version. Sometimes removing and re-adding the email account also resolves synchronization issues.
If your mailbox already contains thousands of duplicate emails, manually removing them can be very time-consuming. In that situation, I found the SysTools Thunderbird Duplicate Remover to be very helpful. It scans Thunderbird mailbox files, detects duplicate emails using multiple criteria such as subject, sender, recipient, date, message body, and attachments, and removes only the duplicate copies while keeping the original emails safe. It saved me a lot of time and cleaned up my mailbox without affecting any important data.
