You need to understand how your Google email account works from the Google end and how they translate that to an IMAP interface. See https://support.mozilla.org/en-US/kb/thunderbird-and-gmail Essentially Gmail is a Frankenstein's monster. It stores mail only in "all mail" and builds "folders" on the fly by searching "labels". When it comes to IMAP folders, it sends multiple copies of mail, one for each "label" that is being converted to an IMAP folder. So perhaps the easiest cure is don't use Gmail use a provider that has a mail service without all of the Google quirks. Otherwise you could just unsubscribe from the gmail / all mail folder in Thunderbird and never see it again. I see lots of folk stressing about storage space. Is it because they are stressing about nothing or did they purchased computers that have basically insufficient disk space for their needs. I have never been able to understand the issue really. My terabyte SSD cost a couple of hundred dollars some years ago and I have never managed to fill more than a third of it.