Duncan is talking stuff no one understands.
He is also preventing your entry into the Establishment. Talk to the person behind the door. Promise to get rid of Duncan.
This is the low-spoiler guide for "Duncan the milk cat" puzzle in Nine Noir Lives. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.