I've seen this happen quite a lot with servers that I'm in.

I've this this happen when someone is not logged into Discord in their browser and is logged in in the Discord app. When they click the server join link, Discord creates a new dangling (no-email-attached) account. However, since they are logged into the Discord app, their real account also joins the server when they launch the app from the browser.

I'm pretty keen on sharing this knowledge because I recently realized I've done this myself, and it took me way too long to figure it out. I "started" using Discord a few months ago, and in one of the servers I joined, I saw an account with an oddly-familiar username that joined a server about a minute before I did. And then I saw the same thing happen in another server! After a few weeks of having a weird feeling about this, I finally found out that in my browser, I was logged into a dangling account I created back in 2018, before I really knew what Discord was. (I think back then I thought Discord was just a voice chat service without permanent accounts.)

I would love for Discord to solve this and a few other identity management-related issues!