Windows 10 Home activated on two machines

[footman]

I have recently built a Win10 machine with new hardware and
purchased a key from Microsoft.
I have just built a second machine for a dedicated purpose with components bought on here in Members Market. In this installation I specified that I didn’t have a key. Now that it is complete when I check on activation it says it is already activated in my Microsoft account.
How can I possibly have 2 machines with Windows 10 seemingly activated when I have only paid for 1?

 

[maddness]

Maybe the previous owner had windows 10 installed, if so then the os is tied to the motherboard, so you're using his... Tell him thanks