There are two ways to share items in Google Reader. You can click the share button at the bottom of a post or you can use the share bookmark.
If you do both then you'll end up with two submissions to your shared items feed.
It's a fair question ask; "Why would you share a single item twice?".
It can happen by accident, of course, it's all too possible that you'll end up on an actual blog page (perhaps following a link from Sphinn or Mixx) and decide to share it. Later on, back in your Reader, you rediscover the same post, notice that it hasn't been shared (or doesn't seem to have been shared) and press the Reader share button.
I use my share button a lot; it feeds Facebook, Friendfeed, MyBlogLog and as well as some internal bigmouthmedia news networking. Sometimes I'm keen to get a blog post shared quickly, so rather than waiting for Google Reader to update, I'll use the bookmarklet on the post itself.