Thanks for your answer. I was thinking along the same lines, that it's somewhat of a security measure.

But if there would be a flaw in SHA256, wouldn't that impact both SHA256d hashed transactions, pretty much the same as SHA256 hashed transactions? It would of course depend on the flaw found, but reasoning that Satoshi choose this as a security measure, we're talking delay here.

Just wondering if the choice for SHA256d is a functional decision, like speed improvement.