A startup is creating a business service for the hotel industry. The service will allow hotels to sell unoccupied rooms on short notice using the startup’s platform. The startup wants to make it as easy as possible for hotels to share data with the platform, so it uses a message queue to collect data about rooms that are available for rent. Hotels send a message for each room that is available and the days that it is available. Room identifier and dates are the keys that uniquely identify a listing. If a listing exists and a message is received with the same room identifier and dates, the message is discarded. What are the minimal guarantees that you would want from the message queue?