This is quite interesting, and I assume a relevant issue if steemit is to succeed.

Something to consider would be Steganography (https://en.wikipedia.org/wiki/Steganography). The following is not a solution to detect whether the author copied content from another source, but it would protect the original authors here on steemit from being accused of plagiarism, if content from here is copied to a different blog on the internet. Steemit could for instance automatically hide some unique id/message in images that are uploaded. If that image is then duplicated and used somewhere else, that hidden message would allow to identify from where the image originates - e.g. first been published here on steemit. Steganography is not an ugly watermark that distorts the image for the viewers, but instead is invisible to the eye and can only be identified if the applied method is known (by steemit itself). All this however, only works if there are images used within the post. It is not applicable to text-only posts.