We investigate the one-triplet dispersion in a modified Shastry-Sutherland model for SrCu₂(BO₃)₂ by means of a series expansion about the limit of strong dimerization. Our perturbative method is based on a continuous unitary transformation that maps the original Hamiltonian to an effective, energy-quanta-conserving block diagonal Hamiltonian H_eff. The dispersion splits into two branches which are nearly degenerate. We analyse the symmetries of the model and show that space group operations are necessary to explain the degeneracy of the dispersion at k = 0 and at the border of the magnetic Brillouin zone. Moreover, we investigate the behaviour of the dispersion for small |k| and compare our results to inelastic neutron scattering data.