Duke is the tenth out of 13 titles in the game. It gives you some advantages over the Marquis.
From the moment you are given the title, the amount of XP and EGO earned in the game is multiplied by a factor of six. In other words, you start earning XP and rewards in EGO six times faster.
Your Daily Tribute and Faction Tribute become six times as much.
You can become a Duke one way only:
The risk of losing the title only comes into play if two conditions are met at the same time:
You will be informed if one of the risk factors occurs. You will have at least 48 hours to meet the requirements. If you don't meet them you might become back a Baronet or lower. That depends on your EGO balance.
Make sure your EGO balance is well above 250,000,000 Ego. This way you will significantly reduce the risk of losing your title even if you no longer is a parliamentarian.