If a man is already married in Jannah, can I pray to marry him in Jannah?

He's married and in Jannah. You're praying five times a day that you marry him in Jannah. Sister, if he's married then let him be married. Focus on yourself getting to Jannah in the first place. That's the most important thing. When you get to Jannah inshallah, then you will have everything that you desire, that Allah will give you, make you eternally happy. But at the moment, if he's married, focus on yourself, focus on your Jannah, and focus on that rather than making dua for somebody else in this particular way.