Are there previous formulations of this quote from George R.R. Martin

The earlier formulation was by William Shakespeare in the play Julius Caesar:

Cowards die many times before their deaths; The valiant never taste of death but once.

Supposedly, somewhere in "A Farewell to Arms", Hemingway wrote:

"The coward dies a thousand deaths, the brave but one?"

"Of course. Who said it?"

"I don't know."

"He was probably a coward," she said. "He knew a great deal about cowards but nothing about the brave. The brave dies perhaps two thousand deaths if he's intelligent. He simply doesn't mention them.”

Expressions of the form "the coward dies a thousand deaths, while the valiant never die but once" go back to 1826, at least, and, as can be seen with the Hemingway quote, the line was subject to being rewritten in numerous ways. There's little doubt that the original source quote was either Shakespeare's line or one from an even earlier author (that Shakespeare likely copped), and the line has been quoted and requoted over the centuries.

There's also little reason to doubt that Martin might have simply stolen this popular meme and modified it to suit his needs. Other folks are welcome to employ their Google-fu to see if they can find an earlier version of the Martin quote, but I find nothing.

Thanks to @hot-licks, a few searches in google books turned up variants much closer in meaning:

In 2005, Marcus Omer wrote - 'A dreamer lives a thousand lives, a hero lives but one.'

This quote is in fact a bridge between the Shakespeare's and Martin's.

Willard Waller wrote - 'when a thousand men live together, then each man lives a thousand lives.'

This one gets to the notion of enriched experience of community, in the same way that St. Augustine touches on breadth of experience through travel.

I'm not yet ready to accept this as a final answer, so please keep them coming.

I've traced the trope to a 2008 interview of Nobel prize winner Mario Vargas Llosa on El Clarín, a recognized Buenos Aires newspaper:

"Los humanos tienen esa curiosa condición de poseer una sola vida, pero al mismo tiempo estar dotados de esa imaginación y esos deseos que los hacen anhelar mil vidas".

My translation is "Humans have a curious condition that makes them hold only one life, while at the same time they are given imagination and desire enough to long for a thousand lives". And Vargas Llosa goes on:

"Mil vidas, una experiencia más rica e intensa de lo que viven en realidad. Y sienten ese gran vacío entre la realidad y el sueño, el deseo. Ese vacío lo llenan las ficciones y la novela es la forma suprema de ellas".

My translation is "A thousand lives, a far richer and intense experience than their real life. And they feel that void between reality and dream: desire. That void is filled by fiction, and the novel is its supreme form."

Since A Dance with Dragons was published in 2011, we could safely assume that Vargas Llosa's quote was at least one of the original sources. My personal conclusion is that there are no originals, only infinite variations. From Vargas Llosa, it should be possible to trace this back to St. Augustine again.

“The person who doesn’t read lives only one life. The reader lives 5,000. Reading is immortality backwards.”

― Umberto Eco

This isn’t exactly what you’re looking for, but what about Emily Dickinson’s poem? Certainly the meaning is similar to Martin’s statement.
There is no Frigate like a Book (1286) BY EMILY DICKINSON

There is no Frigate like a Book To take us Lands away Nor any Coursers like a Page Of prancing Poetry – This Traverse may the poorest take Without oppress of Toll – How frugal is the Chariot That bears the Human Soul –