We propose an approach to unification in modal logics that combines duality and step-by-step constructions of terminal frames. We apply this approach to the modal logics K, Alt1, and a logic that we call KdB. For K, we show how our methods give an effective reformulation of the unifiability problem as a non-trivial decision problem involving neighborhood frames. For Alt1, we show that the problem is PSPACE-complete. For KdB, we establish that unifiability is equivalent to a problem involving de Bruijn graphs.