There will almost certainly be overlap (possibly 100%, possibly one or two differences). We must just be characteraholics because we often meet same characters twice in one day
I definitely laud your sentiment of letting other little girls and boys have the chance. I will say, I did find it a bit annoying that the princesses are out in the open in the lobby--there are a lot of frustrated people who wait in the long windy line only to get to the ticket taker and be turned away. So if you are on the fence, another option is to get the tickets, and if you decide you don't need them, offer them up to someone in the lobby who gets turned away (there were plenty such people when we went, so I doubt it would be hard to find someone to give them to on the day of if you happen to be walking through the lobby at some point when the princesses are there)