One reason that happens is because email providers have their own idea of what folders should be name, such as sent-mail vs sent. Some let you delete the non-standard folder, while others do not. Anyway, on your question, the easiest approach I've seen is to use the addon, importexport. With it, you can highlight a folder and export as eml files and then highlight the other folder and import the eml files. That merges them . If needed, there is an addon, remove duplicate messages, to clear out duplicates.

One reason that happens is because email providers have their own idea of what folders should be name, such as sent-mail vs sent. Some let you delete the non-standard folder, while others do not. Anyway, on your question, the easiest approach I've seen is to use the addon, importexport. With it, you can highlight a folder and export as eml files and then highlight the other folder and import the eml files. That merges them . If needed, there is an addon, remove duplicate messages, to clear out duplicates.

Thanks for the reply. I'm not concerned with duplicate emails. Both folders seem to have the same content. I want to remove the duplicate folder from showing up in the list.

It can depend on the email provider to a degree. For example, Outlook always saves the sent message, so if you have clicked to save sent messages, you might be saving to a different folder. That's normally the case when both have the same content. If one has content and the other doesn't, or has different content, that usually happens when a user sends messages sometimes online and sometimes from Thunderbird, as some email providers have a predefined name for some folders. For more specifics, you may need to post the email provider info and setting for saved sent messages.

David, I have a Gmail account and Yahoo account that have double folders. My other two email accounts do not.

I'm not familiar with Yahoo, but I imagine it's similar to Gmail. With Gmail, there is only one sent folder. all others are just labels, such as Allmail. If you are showing two sent folders, I think you can highlight the account, rightclick and select 'subscribe' and unsubscribe for the extra sent folder.