Do you ever go to StackOverflow twice for the same problem?

It may be worth keeping a notepad, journal, or even a blog that you can document snippets and things you've learned if you're finding that you're using Google or StackOverflow often to try and solve the same problems.

I personally just keep around a handful of markdown files split up into different categories for things I use every few months, but can never remember the exact command or series of commands to accomplish it.

I've also had a few coworkers and friends who have hosted blogs (some public, some private) for the same purposes.

Documenting them in your own words will also likely help with the knowledge retention so you may even find that you won't need to look it up next time it happens.

I think so. I find myself sometimes doing a search, where the top-level link(s) are colored visited, so I must have seen them before.

One of the most common reasons is Python's argparse. I find it easier to consult the documentation when I need it than remember how it works.

Every once in a while the page which gives me the answer is something that I wrote.

All the time. StackOverflow questions are there to stay.

Yes