As others have mentioned, I have many duplicate cobs.
As far as briars, while there are no true duplicates in my collection, I have a 7 day set of Boswells I love. They are all different from each other, but two of the small ones are almost identical and two of the mid size are almost identical. I probably would never change them either. I wouldnt be upset if I had duplicates of two or three of my favorite pipes. Sad thing is, I dont even think if you had the exact same pipe from the same brand, it would smoke the same. Not sure why. Magic probably