I am already logged into Discord, yet it asked me to “claim the account” and then say the email is already registered, how to solve it

There are times when I am already logged into Discord (note: I logged in already), with my user name showing at the bottom left of the window, and then I want to join a certain channel.

Then it says at the bottom "You must claim and verify your account" with a "Claim Account" button. So I thought, I am already logged in with the user name showing, why do I still need to claim my account?

And if I click the "Claim Account" button, and type in my email and password, it'd show an error, "Email already registered" and refuse to go on… So what is it about and how to solve it?