You can form stones pretty much any time. Extra chunks of cholesterol (mmm...) - and these can get lodged in the bile duct even after gallbladder surgery. It just so happens that most stones are formed in the gallbladder and most of them don't become lodged in the bile duct, so it's a pretty rare occurrence for stones to form outside of the gallbladder AND become lodged in the bile duct. But possible (as is pretty much anything else you can think of, in terms of medicine).
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