That's a great question, and I wish I was good enough at math to give you a sophisticated answer. But my thinking is that the entropy you might gain by allowing duplicates is negated by the huge set of weak/guessable passwords you allow. For example, the password "aaaaA1!" is probably more likely to be guessed or used by others than "agkxA1!". (I just checked on haveibeenpwned.com, and the former has been seen 12 times, while the latter has been seen 0 times. Not very scientific I know)
Though this isn't set in stone if someone wants to formally correct me.
Though this isn't set in stone if someone wants to formally correct me.
And libsodium is indeed a pleasure to work with.