This may not be the issue, but check your setting for View>Threads to see you ticked 'expand all threads.' For the sound, untick that at settings>general. The sound comes because it is an incoming message.

Nope, same issue happens without threading (duplicates messages in my collective inbox)

Regarding the sound - I DO want a sound notification for my "legit" single-occurrence emails when they first come in.

You are viewing an individual email in conversation view. This pulls all mails from all locations into a single thread. Your duplicates in green are one from the all mail folder and one for the inbox and one from the gmail generated important folder. That is normal and expected for a gmail account which routinely stores multiple copies of individual emails. It is one of the reasons we generally recommend you do not subscribe to the all mail folder in Thunderbird and personally I simply turned off the important folder as something I did not need.

You second grouping in red indicates two copies of the same email in the all mail folder. That usually indicates the global database is not up to date and needs to be rebuilt. If that is the case, selecting one of those entries will not not display the email.

See https://support.mozilla.org/en-US/kb/rebuilding-global-database

Sounds good - but:

1) I can't see any "All Mail" folder on the left-hand folder pane. (I DO see it in search results when I search for "All mail" - but I cannot access it via the search results... 2) I unchecked "Show in search results" for the "Important" folder - is that what you mean by "not subscribing"? (How do I unsubscribe from these folders? I indeed do not need those copies of the messages displayed) 3) Regarding (2) - is that why the folder was highlighted in blue? Is it because it was regarded as a separate folder in threads and searches?

Thanks!

You are viewing an individual email in conversation view. This pulls all mails from all locations into a single thread. Your duplicates in green are one from the all mail folder and one for the inbox and one from the gmail generated important folder. That is normal and expected for a gmail account which routinely stores multiple copies of individual emails. It is one of the reasons we generally recommend you do not subscribe to the all mail folder in Thunderbird and personally I simply turned off the important folder as something I did not need. You second grouping in red indicates two copies of the same email in the all mail folder. That usually indicates the global database is not up to date and needs to be rebuilt. If that is the case, selecting one of those entries will not not display the email. See https://support.mozilla.org/en-US/kb/rebuilding-global-database

OK, I found the "ALL MAIL" folder and unchecked the show in search results.

Is that what you meant?

You are viewing an individual email in conversation view. This pulls all mails from all locations into a single thread. Your duplicates in green are one from the all mail folder and one for the inbox and one from the gmail generated important folder. That is normal and expected for a gmail account which routinely stores multiple copies of individual emails. It is one of the reasons we generally recommend you do not subscribe to the all mail folder in Thunderbird and personally I simply turned off the important folder as something I did not need. You second grouping in red indicates two copies of the same email in the all mail folder. That usually indicates the global database is not up to date and needs to be rebuilt. If that is the case, selecting one of those entries will not not display the email. See https://support.mozilla.org/en-US/kb/rebuilding-global-database

Right click the account in the folder pane, select subscribe and unsubscribe from the folders you do not want to see.

Removing them from search results will suppress all mail in those folders from all searches.

it is your choice. That is why there are settings for both things. But rebuilding the global database is probably the most important thing needing doing.

I do not see an "Unsubscribe Option" (see screenshot)...

Please highlight the account as Matt indicated. It appears you may have highlighted a message.

Nevermind , found it!