I'm taking a guess here, but in T.H. White's book of that title about Arthur (if I remember correctly), Merlin, who plays a very prominent role in the book (perhaps more so than even Arthur, at least in the Sword and the Stone book), is traveling through time backwards, so to Merlin, Arthur already was king (Merlin complains about life in the 20th c). Merlin knows Wart's future very well, and also knows his own. When he finally takes charge of Wart (Arthur), Wart is only a boy, who Merlin trains to be a King in Arthur's future. In that way, he is, to Merlin (and Wart), both the once and future king.
That is the only famous use of the once and future that I know of, and as it is a well known book, any other use, especially in fantasy, would be referencing that title.
However, in the Book, Merlin does acknowledge that both he and Arthur will come again (a nod to Arthurian legend):
I will tell you something else, King, which may be a surprise for you. It will not happen for hundreds of years, but both of us are to come back.
Some people have used once and future to indicate enduring or perpetual. I can find uses of the phrase after the book but not before it. Maybe someone else will find an earlier use.
I did find this note in Cliff notes:
Merlyn - A magician who has already lived the future, so he knows what is going to happen next. Merlyn is Arthur’s tutor and friend. Arthur’s creation of the Round Table and a more civilized England is largely due to Merlyn’s influence. Although Merlyn is powerful, he is also kind and a little absentminded.
One of the reasons he's absentminded is that he's always confused about time, since he's going through it in a different manner.
Here, he speaks about Hitler in a way Wart might understand:
There was just such a man when I was young—an Austrian who invented a new way of life and convinced himself that he was the chap to make it work. He tried to impose his reformation by the sword, and plunged the civilized world into misery and chaos. But the thing which this fellow had overlooked, my friend, was that he had a predecessor in the reformation business, called Jesus Christ. Perhaps we may assume that Jesus knew as much as the Austrian did about saving people. But the odd thing is that Jesus did not turn the disciples into storm troopers, burn down the Temple at Jerusalem, and fix the blame on Pontius Pilate. On the contrary, he made it clear that the business of the philosopher was to make ideas available, and not to impose them on people.”
