UI code duplication question

I'm trying to design an architecturally sound system but it seems like no matter how hard I try I can't get around duplicating code. I realize no matter how hard I try some duplication will occur but I'm curious how others with more experience would design different pieces of my system.

My system is split into three tiers; data, business and UI. The UI is currently giving me headaches. The UI isn't overly complex but there are some dynamic work-flow processes which need to exist in both a windows form and a web form. Here is an example:

Clocking Employee Into The System

1) Display Clock-in form
2) Get employee PIN from Clock-in form, perform employee lookup.
3a) Employee Found.
__Is the employee already clocked in?
__YES
____Can employee sign into system?
____YES
____Sign employee in
__NO
____Clock employee out
____Return to step #1
__NO
___Can employee clock in?
___YES
____Clock employee in.
____Can employee sign in
____YES
______Sign employee in
____NO
______Return to step #1
___NO
____Display error
____Return to step #1

3b) Employee NOT found.
Display error
Return to step #1

This process needs to exist in both a windows and web application. My question is how do you abstract out a process that requires dynamic user interaction? I've been reading about the MVC design pattern but I'm obviously missing something. Can someone tell me how they would design the process above, or can you point me somewhere?

If some of these steps change I'd love to re-write them in a single spot and have both the windows and web forms update.

Thanks for your help!

Without getting into detail, you can create a DLL that can be accessed by any UI you like. Winforms and WebForms use a completely different set of controls so the UI itself will have to be duplicated, but every event handler in each UI can call methods and properties of types from a common library.