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#11
Is it the same key as used on the other partition is the question?
In that case this may be a method MS has determined to keep two installations from being on the same computer.
This is debated here since both cannot be booted at the same time, but under the EULA I believe you need a second license on the same computer.
If so you can try to save your installation by trying the steps given which have worked in the past if you want to choose it of the two to activate.
You can also copy your files over to the other OS via Explorer and then delete it in Disk Mgmt. or using PW CD. Post up a screenshot of your Disk Mgmt if you want us to assure it is ready to be deleted.