Fail to render the same image twice with a markdown file.

[jbranchaud]

I have a markdown file with some content like the following:

...
Here is some image:

![some image](image01.png)

Here is the same image again:

![some image](image01.png)

When I run that through gimli, neither of the images are rendered. If I remove the second image tag, then the first one is rendered fine. While it may not be the most common use case, it seems to me that it should be able to render the same image multiple times. However, the current behavior seems to be render it exactly once or not at all.