In Crusader Kings II, there tends to be a motley collection of people with a claim on some particular duchy or county. You can press such a claim by inviting one of these people to court, which will allow you to declare war on the current holder of the title; if you win the war, then the guy with the claim gets the title (and loves you dearly). Of course, going to war just to hand a title to somebody else is no fun, unless they end up as one of your vassals once the war is over.
I've found a good way to bring counties into your kingdom: give that courtier a county (so he becomes your vassal), and then, once the war ends, the courtier-you-promoted-to-count will still be your vassal, and that new county will become part of your kingdom.
My question is, How does this work for duchies? If I give the courtier a county (so he becomes my count), then will he be my duke once the war ends? Or will he decide that he's going to take his duchy and become independent? Do I need to give him a duchy if I want him to be my duke after the war? How can I press a courtier's claim for a duchy and make sure that duchy becomes part of my kingdom?