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for quite a few releases. It's known as "single-Instance Store" and was, I believe, around in W2K3 server times. As far as I know, it's not a bought-in or third-party solution.

It only ever appeared as "Windows Storage Server" and was sold as an OEM to hardware vendors offering NAS-like storage boxes. With Windows Server 2008R2 it's just an extra DVD containing the feature (or role, I don't remember) that you install after a normal server installation.
I'm running it on my main data server, which has duplicated music, photo and image files and it seems to work well. I've simply "forgotten it's there".

RS