"Your account has been signed in on another client"

This is possibly the most alarming thing I've seen in my entire life. I have never shared my password with another user, but I immediately changed it when I got the notification. This happened in a Ranked Queue which dodged itself when I closed the pop-up message. Has anyone else ever had this message, and what does it mean? Riot support has been contacted, and I have changed all my passwords and traced sign-ins from all my e-mail accounts.
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