My iCloud email appears twice on the mail app on my Mac

See screenshots.

In the side-bar, in the SMTP list, and in Connection Doctor, iCloud email account appears twice in Mac mail app.

This would be tolerable, but I can't send any email because I get SMTP errors.

I've seen several users report this issue here. They get unhelpful responses like "you're confused", "It's a favourite", "try signing out of iCloud and back in".

I'm pretty sure this is a Mail App preferences/cache issue. Or iCloud account confusion. But before I start deleting preferences and removing accounts, wondering if anyone has seen it before and knows a specific solution please?

I would delete entirely the iCloud account from this machine, then re-add. But I have 200Gb of Photos originals that would need to re-download, and I'm worried about duplicates or lost data.

I note that iCloud mail can be enabled/disabled in Settings> iCloud, Settings>InternetAccounts, and Mail>Settings. If it's just a question of deleting a plist or something, let me know.

Thanks.