It’s best to first double-check that this isn’t a previous booking which was canceled and resold on Booking.com.
Auto-replenishment on closed rooms
You might have had a cancellation, where the room later went back on sale due to the auto-replenishment feature on closed rooms. This is designed to give you more visibility and boost your conversion on Booking.com. However, you can disable this feature on the Extranet (it’s found in Settings in the Property Details tab).
There might be a connection issue between your provider and connected channels like Booking.com. If you think this could be the case, it’s best to contact your provider who can investigate this.