After I gain prisoners from wars, especially after siege of enemies's capital, I prison all of his family and I hurry to the selling them. When I find right price and sell it (it takes few clicks until i find right price), then again pop up appears and tries to sell my prisoner slave. Same person and even I sold her/him. If i sell her with this pop up again, sometimes his owner changes but i still get my money. This happened to me in multiple games. I also not sure about if this happens to normal slaves i just noticed it with war prisoners.
