Mail Sidebar contains a list of email accounts:
Favorites (a collapsible list):
1 for my cable provider
3 for my hosting provider
1 for iCloud
iCloud (a collapsible folder with 11 items)
Flagged
Spam
Smart Mailboxes
a list of the same items in “Favorites” above.
When "Favorites is first opened, “iCloud” shows an ‘open envelope’ icon. At some point, however, that icon disappears.
Question: Why are there rwo 2) separate and identical “iCloud” mailboxes plus the item in “Favorites”? I assume I did something to force all of them to appear, but I do not know how. The second (duplicate) “iCloud” item did not appear until several weeks after I updated to Ventura (13.6.4).
I’ll try to upload a screenshot of my Mail Sidebar:
I think the second iCloud item is actually a folder. If you click on > it will expand to show iCloud inbox, sent etc.
Favourites is a default list of all accounts. You can remove iCloud from it if you like (without removing the account).
“the second iCloud item is actually a folder” I agree. It’s so nice of Apple UI team to make three different ways to use a ‘folder’ icon in two different locations; either to the left or right of the name.
But I still don’t know how that second ‘folder’ got to that position. Perhaps I found a new (and mostly useless) way to duplicate a favorite/account.
Nor can I figure out why the ‘envelope’ icon of the “iCloud” in the Favorites list eventually disappears. That iteration is, logically, just the “Inbox” of that account.
Because of that ‘logic’, I assume(!) the “You can remove iCloud from it” statement you made is referring to second/duplicated “iCloud” (labeled “2”) in the image. One can’t be too safe when deleting things! At least this is on MY iMac not my wife’s MBP!!!
Sorry - I got confused. I thought there was a Favourites folder at the top that could be edited. But that is not the case.
Not sure how the second iCloud folder got there.
It appears that your iCloud account got favorited. If you right (secondary) click on the 2nd iCloud, you should get a contextual menu with the option to remove from favorites. Select that, and it should go away.
Indeed, control-clicking the second iCloud instance (folder), brought up several options. One was “Remove iCloud from Favorites”. Used that and things have been back to normal for a couple of days!
Haven’t found a keyboard command to put an account into Favorites but that may be due to the fact that all of mine are already in there. However, I have sometimes selected multiple items in a window and I think it is possible to simply drag an account, in the last/lower list, and drag it into the Favorites area. I assume that’s how I managed to create this minor problem for myself.
If the Mailbox is not in Favorites, control-clicking the icon in the Sidebar will bring up the item “Add to Favorites”.
The View menu allows you to show or hide a bar of your favorites at the top of the viewer window. I use this and hide the list in the Sidebar by tapping the ‘Favorites’ entry in the Sidebar and tapping the carat to have it point sideways. Note that you can also add items to Favorites by tapping the
‘+’ incon that appears. The Mailboxes’ order in the Favorites bar is the same as in the Sidebar. So, you can change the order by dragging a Mailbox sideways in the top bar or vertically in the Sidebar.
One can’t be too safe when deleting things! 
At least this is on MY iMac not my wife’s MBP!!! 
