I read something written by a native, reliable English speaker, where he ended his sentence and question with a question mark followed by a period.
What he wrote:
[Some preceding text] If you were a horse you could possibly call another horse a horse being, because all living things must have a sense of "self" of existing. Don't they?.
I read about this matter and found that it is not necessary for a period to come after a question mark since the latter serves as the former.
Could it have a specific indication such as:
"The matter is not for any further discussing.", or is it simply a clearer way of stating a rhetorical question, or is it just an unintentional punctuation mistake