When does one go for a double doctorate? I would love to know the circumstances under which people opt for a second doctorate degree.


Could a professor working in a university write up a thesis and submit it to his department to earn a second doctorate?
Do only those who want to remain in the university forever as students go for a second doctorate?  
Are there any general merits/downsides to earning 2 PhDs, if you are perhaps assured of a research career after a good first one?
Generally you cannot get two PhDs in the same field. One could get a second PhD in a second field.

People who want to do research in multiple fields or in a multidisciplinary topic or wish to change fields may obtain multiple PhDs. Alternatively, as you suggest, people who want to remain students forever do that.

The main downside of doing multiple PhDs is that people may not see that you are moving on with your career. There is a career after getting your PhD, and obtaining a PhD is a small step along that career path. Hovering around getting multiple PhDs would be akin to getting multiple bachelor degrees. People may see you as a permanent student, not someone growing into a mature researcher.