I'm stuck on mission 8 -The Grand Palace (spoilers)

[Landman]

I've inflitrated the Palace and stolen the vault key so I've recovered Delilah's soul. I'm trying a non-lethal playthough, I can't figure out which is the real Duke and where the decoy is. There's a guy in a bedroom who had the key that I pickpocketed, but when I try to speak to him he says I've got a great plan and he'll take the place of the real Duke but then walks off and rings an alarm. I've been through the whole level and I can't find another Duke. I've tried choking him out but a red mark appears over his head and the game is telling me to kill him. Where are the locations that the Dukes appear?

[Zekiran]

If you carefully listen to conversations, and look for notes here and there, the guards and employees talk about where the Duke is taking dinner. Sometimes he's in his chambers, sometimes in the 'throne' room. Clean out any unconscious bodies and disable the alarms before trying to talk to the 'duke'. If you get the key to the vault off the guy, that IS the duke. If you get a different key (to a chest) that is the double.

[Shia Luck]

One of them smokes. Overheard conversations and notes will tell you which one. Hiding and listening to him talking to himself should make it clear too.

Have fun :)

[Specific Beef]

Smoking. It's the whole thing. One has a cigar in his hand at all times (the double), and the other does not (the Duke).

I found the real Duke in the Duke's apartment and the fame in Delilah's suite, but apparently this can change.