This happens on a Windows 7 computer, with Office 2016. Set Your Default Programs window shows duplicate Office apps, i.e. Outlook & Outlook (Desktop).
After a long online search, I found a solution, though it's just "a work-around (not as a fix to the fundamental problem)".
The solution is to add the MS Office apps path to the PATH environment Variables. In this case: C:\Program Files (x86)\Microsoft Office\root\Office16