If the single photo in the Untitled Project is the "original", does it not still remain the original even if moved into a larger event? If it does, would I not, by removing it, I also remove all other copies in Aperture? For example, were I to drag the photo of me on the beach into the event named Royston Clarke, there would be to identical photos side by side, one flagged, the other not. Would the unflagged not remain the original?
If you are seeing images in two different projects/events, you have imported it twice and have two originals photos, and maybe several edited versions. When you import an image file to Aperture, the file is stored as the original and you will see a version of the image in Aperture. All versions of this image will be in the same project; so similar images in two projects can only be caused by importing twice and you have two originals.
I thought your problem is, that you have used one of the duplicate photos in albums and books, and one is redundant. Moving photos between projects/events does not delete the original image file and does not remove image versions from your albums. So you can safely consolidate the photos in single-photo untitled projects by dragging them to the projects with similar images. If you now see two similar images side-by-side when you sort by date, you can delete one of them and should pick the one, that is not used in any album or other product.
That is why suggested to flag or color-code all photos that have been used in albums, books, etc. If you see two similar photos in the same event and one of them is flagged, you know, that you can safely delete the version with no flags from the project.