In this paper, we study a dual-LMI-based approach to H-infinity performance limitations analysis of SISO systems with multiple (i.e., duplicated) unstable zeros and poles. The scope includes the analysis of the transfer functions M = (1+PK)(-1) P, S = (1+PK)(-1), and T = (1+PK)(-1) PK where P and K stand for the plant and the controller, respectively. The latter two transfer functions are well investigated, and exact closed-form performance bounds are already known for the cases where the plant has the sole unstable zero of degree one or the sole unstable pole of degree one. However, such exact bounds are hardly available for the cases where the plant has multiple (i.e., duplicated) unstable zeros and poles. To obtain a lower bound of the best achievable H-infinity performance for such involved cases, in this paper, we study a dual of the standard LMI that represents the existence of H-infinity controllers achieving a prescribed H-infinity performance level. By deriving a parametrization of dual feasible solutions and constructing a dual suboptimal solution analytically, we can readily obtain a lower bound of the best achievable H-infinity performance. Keywords: H-infinity performance limitations, dual LMI.