Learn English – Origin of once again

How about "thousands" of instances of its usage? I can't resist the irony! (Add your contribution!)

• If I've told you once, [please consider] I've told you a thousand times. - just now
• If I've Told You Once, I've Told You 1,000,000,000 Times - 2010
• If I've told you once, I've told you countless times - 2002
• Dear Rocker in Constance: If I've told you pinheads once, I've told you a thousand times — anybody who listens to the Beatles in 1995 is not only living in the past, he's brain dead! - 1995
• I told you once, Hopkins, I told you million times - 194?
• If I've told you once I've told you five thousand times - 1941
• If I have told you once I have told you ten dozen times - 1930
• If I've told you that once, I've told you thrice — I've told you a thousand times - ?1927?
• don't you know that I not only told you once but, in response to repeated importunities from you, that I told you several times - 1919
• if I said to you once, don't touch confiscated property, I told you so ten thousand times - 1866
• once is as good as if I had told you a thousand Times over - 1735

As a @FumbleFinger's once said:

It's just a standard speech device that people reinvent repeatedly

It's associated with the Holocaust for this reason:

While the phrase “Never Again” was in use before World War II — it was the title of several early American films — it has, for Jews, become shorthand for an ongoing commitment to remembering the Holocaust. The phrase was first popularized as a Jewish rallying cry by militant Zionist leader Meir Kahane in his 1972 book “Never Again!: A Program for Survival.”

The book of a Zionist leader has made this phrase of special meaning to the Jews. The book was about the survival of the Holocaust attempt of the Jews.

The usage of "Never Again" might have first appeared on handmade signs put up by the inmates, but it didn't gain special meaning until Meir Kahane wrote his book.