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".
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