The introduction of a step discontinuity in one of the arms of a symmetrically coupled microstrip rhombic resonator creates a disturbance in the symmetry of the resonator, and results in the splitting of the resonant frequency. This is evidence of the fact that two degenerate modes are excited in the resonator and experiments carried out verify this phenomenon. Design curves are also presented to show the dependence of resonator performance on its parameters, as well as substrate parameters. This resonator could find applications in double-tuned narrowband microwave filters.