Experiments from literature have shown that the diffusion coefficients of dimers of rhenium atoms on tungsten are larger than those of the single rhenium atoms for the cross-channel configuration of the dimer and smaller for the in-channel configuration. In the present paper, expressions are derived for the diffusion coefficient which explain the above observations. They are based on Langevin equations of the motion of a single atom and dimer on a metal surface, in which the relation derived previously by the authors for the friction coefficient is employed. It is also demonstrated that the diffusion coefficient of the dimers exhibits a periodical dependence upon the ratio between the dimer length and the metal lattice parameter, phenomenon which is called resonant diffusion.