Are you sure to use different, unique "user names" for each account? We set up what you want very often without any problem.

By user names do you mean Account Name? Can you provide a screenshot?

I use a different account name but it still keeps telling me incoming server already used? I think I'll just uninstall Thunderbird and add the accounts again.

Uninstalling Thunderbird didn't help. I still get "an outgoing server already exists with that name" ?

No help here, so looks like I'll have to go back to Outlook.

Uninstalling Thunderbird didn't help. I still get "an outgoing server already exists with that name" ?

A most unlikely outcome as the accounts data is stored in the profile which is not affected by a reinstall. Unlike Windows dedicated software, data is not stored in the registry awaiting corruption.

No help here, so looks like I'll have to go back to Outlook.

If you like. What application you use is entirely up to you.

I would suggest however that you check in account settings > outgoing server (SMTP) and make sure that whatever is already there has the email address appended in the name to make it unique. The wizard used to reuse existing outgoing servers, but this caused issues with Microsoft and Google who liked the user name to match the email address used, so changes were made to the setup to make a new SMTP for each new account added. Sometimes however they just get created as outlook.com so the next one is exactly the same.

You might also want to check in account setting what accounts are listed there. Most folk I have seen with this issue have created a non working incoming account and continue to try and add it via the new account wizard with exactly the error you are mentioning.

No matter what I do I simply can't add another account with the same server settings! Utterly ridiculous.