(I was able to reproduce that) When you fight in a tile of a city, your sovereign will automatically enter the city. And when you click on the city you'll see two medallions for your sovereign. Only one of those two medallion can be used to kick the sovereign outside.

(I wasn't able to reproduce that) But later in the game I got some strange behavior : The sovereign I was able to kick outside disappeared and the other sovereign (I couldn't move before that) became the only sovereign.

How to reproduce : build a city next to a spider web, fill every tiles around it. Wait outside your city near the web that a spider will go on one of your tiles next to you, then attack. After the battle you'll be automatically sent in your city. With two sov ... Ubiquity for the win !