@Karen138
I'm not aware of any implicit Airbnb rule around this.
They do have a page ( under Hosting and Hosting Responsibly) referring to country laws.
So whoever may rent out their own or rental property for shorter periods of time according to local regulations may do so.
Also, anybody designated by an owner to do this for them and in their name.
It seems strange that any other person than the owner (or renter allowed to do Airbnb rental) would have the key to that propery?
If it's a second listing as a phishing scam, you must definitely flag them to report their illicit scam to Airbnb.
Also if they are using your photos to advertise a property they have no right to. When reporting/flagging somebody by clicking on the little gray flag, an option pops up in which you can state why you are reporting them.
Hope this helps.