General   (General discussion, talk about anything.)

I believe, it is to do with going beyond the required study of "Doctor" and becoming a specialist in a particular medical field.
A person (Mister...) in the medical field tends to hold the title "Doctor".

In some countries, a person can use a number of titles together; therefore, can hold the title: Doctor Doctor...
In the UK, it is customary to only use one title with the necessary qualifications after the name.

But, to check this, there is Google. Although, Google does not always provide the information required.