Create a multiple choice quiz and go to add a question.
You have a lot of html areas in the same question editing page.
Go to add in an html area (the one you like) a picture uploading it from your local hard drive.
All works fine.

Now go to a different html area and try to add a second image still uploading it from your local hard drive.
You can locate it but at the upload time you get the error specified in the attachment.

If you try to make the same procedure, for instance, adding a course label where ONLY one html area is available in the editing label page and you try to upload two pictures from within THE SAME html area the problem doesn't exist.