I very much doubt that friends of a guest would just 'take it upon themselves' to find out the address of where their friends were staying and then come and knock on your door asking to look around your Airbnb @Audra-And-Bret0
Much more likely this was done at least with the knowledge of, and probably at the request of the guests.
This has never happend to me in over three years of hosting and hundreds of guests.
For it to happen to you twice would be highly unusual.
I hope you didn't let them in.
If this happened to me I would ask Airbnb to cancel the bookings.