  In this paper, we study the singularities of spacelike constant mean
curvature one (CMC 1) surfaces in the de Sitter 3-space. We prove the duality
between generalized conelike singular points and 5/2-cuspidal edges on
spacelike CMC 1 surfaces. To describe the duality between $A_{k+3}$
singularities and cuspidal $S_k$ singularities, we introduce two invariants,
called the $\alpha$-invariant and $\sigma$-invariant, of spacelike CMC 1
surfaces at their singular points. Moreover, we give a classification of
non-degenerate singular points on spacelike CMC 1 surfaces.
