I managed to have a success closure on my purchase with Kinguin.net.
I logged a problem with Kinguin and at the same time logged a dispute with Paypal. Kinguin requested me to prove that the product key was indeed used. They provided me with a Microsoft webpage to activate the product key.
Going into the Microsoft webpage I did an online chat with the Microsoft Supporter. I provided the product key and the person confirmed that it was activated previously. After some pleading the person was nice to further provide the month of activation. Using the screenprint I sent the information back to Kinguin.
They accepted my evidence and refunded me this week. I don't know if using PayPal helped in this case but just happy this issue is resolved successfully. I just saw my amount being reversed at PayPal side.