  The question of when two skew Young diagrams produce the same skew Schur
function has been well-studied. We investigate the same question in the case of
stable Grothendieck polynomials, which are the K-theoretic analogues of the
Schur functions. We prove a necessary condition for two skew shapes to give
rise to the same dual stable Grothendieck polynomial. We also provide a
necessary and sufficient condition in the case where the two skew shapes are
ribbons.
