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The question is related to Outlook + IMAP + Gmail: Remove duplicated e-mails
GMail filters incoming e-mail and apply labels to it, which means that the mails are sorted in the respective folders.
There is one big problem I noticed recently. While the local email client (Thunderbird) is running, every email for which filters apply gets duplicated: One mail remains in the inbox while the other is in the (accurate) folder. The first duplicate disappears once I sort it into the folder manually.
There is no problem when the Thunderbird is not running. The solution to the other question is quite unappealing: Stop syncing certain folders. I thought, 2.5 years later there might be a solution to this problem, even if it's one that only works with Thunderbird.
I have a similar issue. I forward email from one gmail account to the other, then use thunderbird to auto move the email to a folder. If done so, I get 2 emails. Never was able solve it. – LPChip – 2015-04-02T21:35:11.057