To Fix Office Product Key is Already in Use Problem, there are two points that we want share regarding this issue. Hopefully you will get your  answer.

• If you have purchase Office from the retailer then retailer might have activated your key but forgot to give the credentials of that account. In this case you have to go back to the retailer and tell them to give my credentials of the key.

• If retailer has not activated your key, then you can contact Microsoft technical support team to have your key deactivated and they will give you new key. 

Things to remember:

1. Your product key can only be redeemed once. It isn’t meant to be re-used or shared. 
2. For Microsoft 365 Family, If you want to allow other users to install Office, You cannot send them the 25 digits product key because they will see error the product key is already in use. You can send them the credentials of your account to install Office on family members device.

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