Rather than double book, I say to the person asking about the unavailable appointment "I'm booked then but I haven't seen this person before, would you like me to contact you if they cancel or don't show?"
Sometimes they say no, sometimes they say yes but have sorted someone else by the time I invite them to book but a lot of them tend to be in the area and able to get to me within 10-15mins or so.
I try to be honest with clients wherever possible, I've been on the receiving end of his kind of thing at a beauticians and it infuriated me. You're not going to end up with any less clients by being straightforward with them.
In all likelihood double booking will lead to negative feedback somewhere.