We demonstrate a high selectivity rectangular optical filter based on stimulated Brillouin scattering (SBS) effect in fiber with bandwidth from 1 GHz to 3 GHz. By using pump-splitting dual-stage configuration, the filter selectivity has been improved by ~15 dB without any distortion of the shape, which is well controlled by using precise digital feedback compensation and nonlinearity management. We also analyze the noise performance of both the single-stage and dual-stage filter by evaluating the signal-to-noise ratio of the amplified signal using coherent detection. The results show that for achieving higher selectivity and improved noise performance, the dual-stage approach performs better than the single-stage architecture. The inherent tuning flexibility with improved performance makes the proposed SBS filter one of the best candidates for microwave photonic applications.