(Spoilers)
Actually, Duncan Idaho does not stay pretty much the same throughout the Dune saga. During the final known times in the Dune story (Chapterhouse: Dune), when the Bene Gesserit and the Honored Matres became one sisterhood, Duncan Idaho was a wildly capable being.
This last known Duncan Idaho ghola was a mentat, a male sexual imprinter and also had a limited form of Other Memory just like the Bene Gesserit had. This Duncan Idaho was created using cells from all past incarnations of Duncan Idaho, and was trained by the dirty Tleilaxu to be a male master at sexual bonding. In fact, when Murbella, a talented Honored Matre tried to sexually bond Duncan Idaho, Duncan managed to sexually bond her too, creating a mutual sexual addiction.
In the end, this Duncan Idaho took a no-ship and scattered to a random location in the universe.