Microsoft 365 - Unknown Installations

Did you purchase from an official Microsoft reseller (& the order was fulfilled by Amazon) or a third party seller?
If a third party seller then the key may have been sold to others, perhaps you were the first to purchase the key so it is registered to your account. While the others have just installed the program & used the same key so those devices have appeared on your account.
Tbh I don’t know if this is how it works but is the only thing which would explain what’s happened.
Can you remove those other devices & see if they return?

Vidi said:

The package was bought on Amazon and the seller appears to be Amazon in the form of Amazon Media EU S.à r.l.

That's good news. You can discount any reasonable notion of having been sold a product key that was also sold to others.

Whatever the explanation turns out to be, it will not be that.

Denis

Can you provide the Amazon link for the product?
Have you removed those other devices?

Vidi said:

There is something strange about this: when I find my order on Amazon, it says across the top that I have purchased it two times.
This is not correct I have only purchased it once.

I suggest you contact them to see what the problem is & to make sure you have not been charged twice. You can do it through Chat or phone. I think phone would be best in your case.
www.amazon.co.uk/gp/help/customer/contact-us?

It would also seem to be worth checking your recent Orders to see if anything else has been ordered on your account that you don't know about
www.amazon.co.uk/gp/css/order-history?

Best of luck,
Denis

I can confirm Office is genuine which is good to know.
As you’ve removed the extra device hopefully it’ll not return.
Check under your Amazon Orders to see if was purchased twice.

Do you run any Office software on an Android or iDevice which may have upgraded? Does the person for whom you installed the software use any of these devices?