It’s best to first double-check that this isn’t a previous booking which got cancelled and was therefore resold on Booking.com.
Auto-replenishment on closed rooms
You may have had a cancellation where the room has later gone back on sale due to the ‘auto-replenishment on closed rooms’ feature on Booking.com. This is designed to give you more visibility and boost your conversion on Booking.com. However, you can disable this feature in the extranet (it’s found in ‘Settings’ within the ‘Property’ tab).
There may 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.