double booking *** to be cancelled please

This is partner forum not help desk.  You should try and get guest cancel.  Message and apologise and ask them to cancel and try and offer somewhere local that is comparable to you.  Then go into Reservation, click Request to Cancel.  Guest to confirm.  If you get booking.com to do it they will charge a relocation feel.

Contact booking.com help team using Inbox icon on Extranet (Option booking.com messages) or give help team a call see link for local phone numbers. 

https://partner.booking.com/en-us/help/where-you-can-reach-us

Try clicking the link for 'where you can reach us'.

Again - this is not the help desk !!!

Click on this link and it will give you contact phone numbers around the world.  

https://partner.booking.com/en-us/help/where-you-can-reach-us