I think you have it backwards. Tons of "content" on StackOverflow is copy/pasted from another source, often some experts personal blog or a niche site, without sourcing to the original.
I think I've seen an unsourced copy only once, I think it was a new answer where nobody else had seen it yet and I edited in a link to the source. But this is many years ago, I don't remember the situation exactly (and I visit stackexchange sites a lot, more than most people I think, since it's the only social site where I permit myself to go to during work hours).