Why am I getting requests for housesitting when I am already booked?

I have an upcoming house sitting booking for the 20-29th. And yet I just got another request to house sit on a night that is during those dates--why am I still getting that request? I can't housesit two houses at once. I don't want to change my availability because I could still do drop-ins during those dates, but I don't want to keep getting requests that I have to turn down.