Duplicate uploads are only checked on a daily basis and per IP, if you and me upload the same image at the same time, neither of us will get the duplicated upload message.
I'm not sure if we should show the image URL for those, like fool them with the image URL so we make them believe that the image got uploaded but actually we just fetched the already uploaded image. It makes some sense to me to just give you the link and everything.
By the way, the dupe checker can be disabled at dashboard/settings/image-upload