When I freshly turn on my computer and reach the log in screen, normally, when I try to poweroff afterwards, the dialog only shows
Power Off
This system will automatically poweroff in 60 secs
Cancel Restart Power Off
I'm not exactly sure what I did recently, but now it says
Power Off
This system will automatically poweroff in 60 secs
Other users are logged in.
<My username>
Cancel Restart Power Off
which seems to suggest that somehow "another me" is already logged in even before I've logged in. How is that happening?
I noticed the same issue on my machine running Ubuntu 20.04, which started today. The only new software I have installed is NoMachine remote desktop client.
I found this page on the NoMachine forum: https://forums.nomachine.com/topic/other-users-are-logged-in-when-i-power-off-or-restart-centos-7 where a user complained of a similar issue. Linked in that thread was this trouble report page https://www.nomachine.com/TR04R09618 confirming this to be a known bug with NoMachine server.
If you have recently installed NoMachine, this is likely why you are seeing "yourself" logged in. As it says in the forum post, you are safe to ignore the warning until the bug is patched. If you haven't installed NoMachine, then I do not know what may be causing it.
I had that issue two times as well just after applying updates. The restart resolved it.