Thirsties were about first and are sized xs s m l etc and the duo are their version of a one size but two sizes if that makes sense?!
Duo wraps have poppers down the front to adjust the size and you have a size 1 and size 2.
Thirsties ( non duo) have no poppers and their size cannot be adjusted.
Other than that I don t think , well can t see any difference in the ones I have.