This might sound odd, but what local folder are duplicated. Things like inbox set etc are Normal.

Thanks Matt for the reply.

I understand that the standard set of half a dozen or so folders will always be created. These are not the issue.

If you right clic on the account the pop up shows an option to "New Folder" on account1 I have used this feature to create many additional folders into which I move eMails I want to keep and to organize my work flow.

When I create account2 on the same eMail provider (rogers) in this case, all of those created folders are duplicated from account1 onto the new account2

This only seems to happen when adding a second email account on an existing eMail provider.

I have signed on to the rogers web email site for account2 (which is actually Yahoo Mail) and verified that these extra folders DO NOT already exist.

I have also created both of these accounts on my phone (A Samsung S25) using Samsung eMail and the folders were not duplicated to account 2.

So, that points me back to Thunderbird as the culprit.