Former United States Presidents are typically still called "President So-and-
so" even after they leave office. It's a name so snazzy, so prestigious, and so seemingly singular that it never goes away.
However, in the case of Bill and Hillary Clinton, as in the case of a few other US Presidental dynasties, the title of POTUS may not be so singular.
If Hillary wins the White House, this will be the first time the President is a spouse of a former President, but there have been a few times Presidents have shared a last name. And when this happens, typically people refer to the former President by a nickname or by their full name:
If Hillary Clinton is elected president in 2016, the nation won’t just have its first female president. We’ll also have our first male spouse of a president — and the need for a title other than “first lady.”
Late night talk show host Jimmy Kimmel asked the Democratic presidential candidate what Bill Clinton would be called in her White House.
“It’s a little bit more complicated with him because people still call former presidents ‘Mr. President,’ so I have to really work on this,” Clinton said on “Jimmy Kimmel Live” on Thursday night.