The disappointing truth is that we simply don't know. Satoshi never explained it. For SHA-2 it can be used as a mitigation against length-extension attacks¹, and this seems like the most likely explanation, but it's just speculation.

¹ https://en.wikipedia.org/wiki/Length_extension_attack

Would length extension impact bitcoin? The messages aren’t secret afaik.

a that’s the botcoin hashing algorithm.

I figured that part out, I wonder why it's being doubled.

That was the way satoshi wanted it. No other explanation is known.