The idea is that he was once king, and will be again. As far as I know, T.H. White did in fact coin the English version of the phrase for his Arthurian book The Once and Future King, but you'll occasionally hear it adapted for other uses ("ladies and gentleman, the once and future champion!"), presumably as an allusion to the book.
A play on The Once and Future King, a series of novels about the legendary King Arthur. But the real star is John Smith as Falstaff, Shakespeare's once and future comic rogue. The once and future icon of the puppet world, Elmo, is recognizable in just about every country on the planet.
On “Arthur, the ‘one-time’ and future king” I honestly don’t know why people today so often seem to forget to use once and twice instead of one time and two time (s), but it can sometimes sound really odd when they do so, like what happens to the once-and-future king of the Britons under the new regime.
In British History/Folkloric tradition King Arthur, who some scholars argue has his roots as a celtic god, is sometimes referred to as the 'once and future king', perhaps you can have a Once and Future King.
It is more familiar in 'The once and future king'. These words are often called 'peripheral adjectives'; 'non-semantically-predicative adjectives' is a more logical term (a future king is not a king).
Using the phrase to refer to exactly the same thing/person in both instances probably started out as a play on words ("The once and Future King", so to speak), but seems to have degenerated into use by people who don't appear to have any clue of the original phrase or what it meant.