There are several posts about enabling Data Deduplication in Windows 8 out there. I recently upgraded my Windows 8 laptop to 8.1, and did of course want Deduplication enabled after the upgrade as well.
Of course this is not supported in any way, but it is a very nice way to save some precious disk space on your lab machines.

The process is basically the same as in Windows 8, the only difference is that you of course need to use the cab files from Windows Server 2012 R2.

If you don’t have a 2012 R2 box nearby, I’ve uploaded a copy of the cab-files my SkyDrive (http://sdrv.ms/1aZsWgk) so you alternatively can get them from there, if you dare. (NO WARRANTIES! :))
Otherwise you should look after the following files on a Server 2012 R2 machine:
Microsoft-Windows-Dedup-Package~31bf3856ad364e35~amd64~en-US~6.3.9600.16384.cab
Microsoft-Windows-Dedup-Package~31bf3856ad364e35~amd64~~6.3.9600.16384.cab
Microsoft-Windows-FileServer-Package~31bf3856ad364e35~amd64~en-US~6.3.9600.16384.cab
Microsoft-Windows-FileServer-Package~31bf3856ad364e35~amd64~~6.3.9600.16384.cab
Microsoft-Windows-VdsInterop-Package~31bf3856ad364e35~amd64~en-US~6.3.9600.16384.cab
Microsoft-Windows-VdsInterop-Package~31bf3856ad364e35~amd64~~6.3.9600.16384.cab

After you’ve extracted the files from your 2012 R2 box (or downloaded them from my SkyDrive), go ahead and run the commands as below:

dism /online /add-package /packagepath:Microsoft-Windows-VdsInterop-Package~31bf3856ad364e35~amd64~~6.3.9600.16384.cab /packagepath:Microsoft-Windows-VdsInterop-Package~31bf3856ad364e35~amd64~en-US~6.3.9600.16384.cab  /packagepath:Microsoft-Windows-FileServer-Package~31bf3856ad364e35~amd64~~6.3.9600.16384.cab /packagepath:Microsoft-Windows-FileServer-Package~31bf3856ad364e35~amd64~en-US~6.3.9600.16384.cab  /packagepath:Microsoft-Windows-Dedup-Package~31bf3856ad364e35~amd64~~6.3.9600.16384.cab /packagepath:Microsoft-Windows-Dedup-Package~31bf3856ad364e35~amd64~en-US~6.3.9600.16384.cab

dism /online /enable-feature /featurename:Dedup-Core /all

If everything works as intended, you should now have Deduplication enabled on your 8.1 machine.
You can verify this in add/remove Windows features. (optionalfeatures.exe)

To enable it for a specific volume, run the following command in an elevated PowerShell prompt:

Enable-DedupVolume -Volume D:
Set-DedupVolume -Volume D: -OptimizeInUseFiles

To force a Deduplication job, simply run the PowerShell command as below (deduplications runs well on open files now as well, as long as you’ve provided the “OptimizeInUseFiles”-switch…):

Start-DedupJob -Volume D: -Type Optimization
You can monitor your DedupJop with the cmdlet Get-DedupJob.

To verify that everything looks alright after the first job, you run the cmdlet:
Get-DedupVolume -Volume D:
Hopefully you will also end up with a nice deduplicated data volume as in my screenshots below:

Happy Deduplication!

/Johan