Okay, it's kind of difficult to explain in a title, so here goes. Imagine you inserted a URL into your post using the reference syntax, and then use that URL multiple times; the most common scenario for this being embedded images that link to themselves.
Diagram ([click to open in a new window]): [!] : http://i.stack.imgur.com/n0rVy.png
However, if one then inserts another image using the imgur upload function, the markdown looks like this:
Diagram ([click to open in a new window]): [!] ![enter image description] : http://i.stack.imgur.com/n0rVy.png : http://i.stack.imgur.com/n0rVy.png : http://i.stack.imgur.com/n0rVy.png : http://i.stack.imgur.com/uKite.jpg
The image is added, but for some reason the existing reference is duplicated for each time it's used in the post body.