@Dean700 I've been reading a thread on another hosting forum of this happening to other hosts. Seems to be glitching out. Contact Airbnb and tell them.
However, are you saying the dates the guest entered on the request shouldn't have been showing as available, or the guest simply asked in his message for those dates? Because guests can ask for you to cook them a gourmet dinner if they want, they can ask for anything.
And rather than decline and ding your acceptance rate, if the guest actually entered those dates on the request, explain to the guest that there seems to be a system calendar glitch- that those dates should have been showing as unavailable and ask him to withdraw the request so he is free to look for another place that has those dates open.