But aside from APL and a few Haskell coders, who isn't programming in ascii? I mean, sometimes i chuck an emoji into a comment but I don't think that really counts.
And how could programming in ascii have solved this bug? The problem is that the strings were compared without giving any thought to what "are these strings equal" is supposed to mean.
The only general solution to this kind of bug - that is, to considering distinct emails to be identical - is to have a special function that can check if two emails are identical according to the spec. This function will be different than determining if two names are the same in an American database where dotless and dotful i are the same, and that function will also be different than a function determining if two names are the same in a Turkish database where dotless and dotful i are different. Although I don't know why you're trying to compare strings for identity, you're probably just after similarity anyway - how often do we convert strings to lowercase before we compare them.
Generic string equality functions are always the source of bugs, since there's no useful, single uniformally applicable definition of string equality.