You probably know that Google Reader has a "Share" option which puts a blog post into your own "Shared" feed so friends who subscribe to that can see what you've been reading.
And you probably know that recently they added a "Share with note" option that lets you... well, add a note when you share it so your friends can see what you think about what you've been reading.
But what you mightn't know is that if you select both "Share" and "Share with note" it goes into your shared feed twice (once with and once without the note). This is a bit stupid, but there you are. To stop it happening just don't select "Share"; selecting "Share with note" all by itself is sufficient.