The insert image feature does not recognize images that are already hosted on Imgur, so when you try to use the toolbar to insert one of these, it gets duplicated on their website.
For example, I took a snapshot of a question and pushed it directly to Imgur. I got my URL here:
Now, if I click the toolbar button, select "from the web", and paste the URL in, it (or Imgur or somebody does this, I'm not sure) re-uploads the image to the host.
If the insert-image function of the editor was to simply recognize the domain of the URL as Imgur, this double-upload could be avoided.