I have Windows 7.
When I am typing a message then some words are duplicated. The post looks fine.
It happens only on this website. It has happened for a while (I got Win 7 a few months ago).
I've seen this before with the word wrap on certain browsers - maybe worth trying a different browser and see if that duplicates the problem. Also does it post the correct format up if you press submit or do the duplicated words appear in your post as well ?
*edit - noticed you'd put that it looks fine on the post