in my experience, nothing to do with case sensitivity, or discriminators, or special characters, or anything else, it's that people will have multiple identical duplicate accounts, whether because they've forgot the password (rip), had them stolen (lmao), or, and this is the really damning thing: accidentally created a new account every time they've ever joined a server because the flow for clicking a discord invite link in your browser really really tries to shepherd you (or at least it used to, I haven't done a lot of server-joining lately) into creating a new account and it takes some like brain ninja computer genius attention-paying to realize you don't actually have to do that but there's some tiny little bullshit other thing you can click to not create a new account but to join with an existing account
as more and more people have gotten more and more used to using discord on a regular basis this happens a lot less than it used to, the first few years after most of the communities I'm in started migrating to discord it was not uncommon to have 4+ duplicates of the same person, basically every time they restarted their computer they would end up with ANOTHER discord account. nowadays it's mostly one or two dead extra accounts
note that I'm not talking about ~parachutes~ or sockpuppets or anything, most of these accounts never get logged into again, they just sit there, forever offline
honestly the server i took those examples from would have been even worse except for the time i tyranically employed my brief tenure with mod privs to start purging obvious duplicates, this was deeply unpopular for shitting up the admin log or some bullshit so now here they sit, forever
can't wait for all these dead accounts to be autoconverted to firstnameBunchanumbers because that's definitely more memorable, or even funnier, to remain as zombies, forever parked on the name the holder really wants, because they accidentally made an account and abandoned it when they created their second account (which they kept using) by accident
honestly there wasn't really a point to any of this it's just almost 4 in the morning and i'm mildly annoyed about computer