When removing image instance from xinha using the image picker, remaining instances are no longer valid

Reported by slawek, see http://hippo.2275632.n2.nabble.com/Xinha-bug-insert-image-two-times-and-delete-one-of-them-tp6916002p6917197.html

Here is more accurate description of this bug on gogreen:
1. create news document.
2. in description field:
add image (eg. /gallery/news/Toyota.jpg)
new line
add again the same image (eg. /gallery/news/Toyota.jpg)
select first image
by "Insert/Modify Image" I remove image.
3. Save & Close document (before this I add the title)
4. Publish (now you can see in Description there isn't any image)