My Mini Dock does this to some extent too, though it more just doesn't release either side fully. The way around it seems to be to pull the handle up to it's fullest extent (towards the PowerBook) then wiggle the dock from side to side. Be careful though as the two clips that hold the Duo are plastic and not very strong looking.

If you get really stuck then try undoing the torx screws underneath the Mini Dock and getting at the locking mechanism that way. It should be fairly easy to manipulate it with a screwdriver or something similar.