booking.com made a double booking for my property.i dont know how to handle this

You have posted this on the partner forum so we cannot help with this. 

If it is their mistake you should 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