You need to choose the baron Harkonnen's ending and then give Alia some of the baron's lollipops.
You then have the option of asking her to stop sucking on them so lewdly. From there, choose the appropriate dialogue (There's only two choices, you should know the obvious one to pick.) and after that, you'll then unlock the scene with Alia and Duncan's ghola in the showers.
It's just her giving Duncan a handjob in the showers while seated in front of him while he's seated on the floor as well.
I will say it's drawn very well, and the animation's better than what's seen in other ingame scenes.