"File not uploaded, duplicate file" in email broadcast

I’m trying to upload a ‘How To’ email for our clients and for some reason my pictures keep getting denied. The response reads, “File not uploaded, duplicate file”. They are not duplicates. This email NEEDS to go out today.
Any reason this might be happening to me?

Although it may not be a duplicate image, it may have a duplicate name of an image that is already uploaded.
Try renaming the image an uploading and see if that works.