Legend theme function doesn't get called if atom added twice or re-added to ckeditor

To reproduce copy theme function function theme_sdl_editor_legend($variables) to template.php and make a change to it.
Now when dragging an atom into wysiwyg I see that the legend/caption has the change I just made in the theme function.
If I add the same image again or delete it and re-add I don't see the change, it's using the original theme function.
It works fine if I just add a bunch of different images, only happens when adding the same image more than once or deleting and re-adding the same image.