Why do people post the same thing twice?

Mostly because of the fear of it not getting read and in turn leaving it unanswered. Especially when you go to the home page, dozens of questions get added every second meaning it's quite easy for one question to go by unnoticed. This doesn't happen as frequent when simply browsing a category.

What a lot of people don't realize, however, is that a lot of regulars (including me) browse categories rather than just the homepage when looking for questions to answer. So it's pretty safe to say that there's a 90% chance that your question will get answered. Provided you're not an obvious troll.

EDIT: Hmmm... What Tinkerbell says actually makes a lot of sense :P... Don't mind me *walks away quietly*

Sometimes when you press submit but then go back before it loads and post again, it will post twice.

Maybe their fingers stutter.