Through some convoluted process involving trying to move my data file I
appear to have ended up with two Personal Folders in the "All Mail Folders"
pane and one data store. I can't "Close" the second copy (it says 'The
operation failed. An object could not be found'). New items are only added
to one of the Personal Folders'

How do I delete the duplicate Personal Folders file/object?

Many thanks.