Why do I get "already logged in" error when I try to join a server?

Someone may have found out your password and logged on. This means you need to go to Minecraft.net and change your password. Then it should be fine, if not I don't know what happened

Someone may have compromised your account for their benefit, you can try changing your minecraft account password from minecraft.net and if the issue continues, you can try changing your email (the one you used to make your minecraft account if you have several) password. And if that doesn't work, it is most likely a bug on the server, in which case I don't have the information to help you with.

Edit: The "already logged in" error is not a vanilla minecraft error, in which case it may be a bug with the server especially if there are plugins on the server, you can try removing the plugins on the server temporarily and see if that works.

If you've tried multiple times at different times, it suggests that either you have someone on your account that never sleeps and really enjoys playing that server, or something has gone wrong with the scripts in the server and it reckons you are active all the time. This may occur if your device shuts down instantly and unexpectedly. What I suggest is that you debug the game, or wait for the next edition to come out and the update should fix it. If that fails, delete and re-download the game, start the server again, and if all that fails buy a new account.