Check-In Error Message: 'You cannot save this worksheet with the same name as another open workbook'

When trying to check-in a document the user is getting a message that:

'You cannot save this worksheet with the same name as another open workbook or add-on. Choose a different name, or close the other workbook or add-on before saving."

The user is on Windows-8 and using SPO and Office-2016.

If user explictly saves the document (Control+S) then the check-in works fine without the above error.  Expectation is that Check-in should give appropriate message during Check-in if the user has not saved his changes.