During the Credits
There are no extras during the credits, however, they are quite long as they list all the developers that have worked on the game over the years.
After the Credits
At the very end of the credits there is an achievement/trophy unlocked that says “Special Thanks”. After the credits there is an additional scene, check out below for more details and spoilers.
The extra scene shows a “live” press conference with Duke Nukem being broadcast on the news as a special report. A reporter asks Duke what he’s going to do now that he’s saved the world and he responds by saying that he’s going to “run for president, hail to the king, baby.”