OMG, two Windows with one product key???

Can you clarify that - do you mean you are dual booting Win 10 Pro with itself- two installations of Win 10 Pro on the same PC?

Note that if you have already installed Win 10 on a given device, and then clean install it, it will (or should be) automatically activated. Presumably that's what's happening with your 2nd installation.

There is this possibility:

but before you rush in, that may then apply to both. So don't try that.

If you are aiming to have two separately licensed installations of Win 10 on the same PC, I would suggest you find your area MS activation phone number, and give them a ring.

I'm sure you'll get some expert comments from others here.

jimbo45 said:

Ms perhaps is becoming "Tescofied"
BOGOF --> Buy one get one Free !!!!
Will I get Clubcard Points.
jimbo

- wait for them to double up and triple boot....

I would suggest you find your area MS activation phone number, and give them a ring if you want a definitive answer, unless someone here gives one. Check the license terms- does it say anything about how many copies you can activate?
Microsoft License Terms

It is tied to your hardware id, but changes in significant parts are permitted. I can't comment on the legality.. see the license terms.

It seems you now get a digital licence even if you use a key to activate Windows 10. That is why it activated the second install.

Strictly, you are contravening the EULA, as you should only have on version on pc at any one time.

Personally, I cannot get emotive about that, as you can only ever run one version at a time i.e. it is not as if you are installing it on two separate pcs using same key which is piracy.

I certainly do not advocate piracy but, in your case, all you are "guilty of" is a minor transgression of the EULA.

Go to Settings > Update & security > Activation. As already mentioned, it likely shows as "Activated with a Digital license". The second install of 10 Pro on the same hardware is activating with the DL the first install got. The activation server doesn't know its two separate installs. It does violate the EULA though. If you want to be totally legal with a clear conscience. Boot up to the second install, then go to Settings > Update & security > Activation. Then click change product key and enter your second product code that you haven't used. Keep in mind that this may just switch both installs the this new key? You will have done due diligence though. Or don't do anything and wait and see what happens? I'm thinking they will both stay activated with no issues. I'm with cereberus, you can only ever run the one install at any one time so I have a hard time getting to bent out of shape when things like this happen. In this case its not your fault the second install activated all on its own.