This is such an interesting thread! I like Rachy's guidance of stopping at two. For these jeans I don't know if I would duplicate unless I could get a different wash ... As I don't wash jeans very often. But this depends on your usual laundry cycle. I like the suggestion you got to test drive these for a while first. But I don't think two pairs of a great fitting jean is excessive.
For myself, I think I tend to dupe too much ... Even my BR magic sweater, I got three and tend to wear only two. I think one alternate of a perfect piece is my magic number!