According to hadith a jannati will get whatever he wishes in Jannah. Does that include nikkah to a girl whom he wished to marry on earth but who got married to someone else by Allah’s will ? Will Allah create another similar person for the Jannati?

You would not get the same girl that you loved in this world, since she is married to someone else. But you may ask Allah Ta`ala another hour (jannati woman) that ressemble exactly the one Allah created in this world and that you loved her. Furthermore, know that you would not feel bad in jannah against anyone and whatever misery and missed you had in this world would vanish on the first gaze of garden of jannah and its women.