With the increasing penetration of nonlinear loads into distribution system, stable operation of power distribution system suffers challenge by harmonic voltage propagation and resonance amplification which is also known as whack-a-mole phenomenon. However, until now this phenomenon has not been well investigated and discussed. This paper starts from theoretical analysis of harmonic propagation and how it is triggered when DG unit interfaced to the grid. Moreover, harmonic damping performance of various types of impedance seen from DG at selected frequencies are analyzed and compared by introducing microwave transmission line theory. In addition, this paper proposes a control algorithm with DG unit where virtual impedances at selected frequencies are individually designed to mitigate the harmonic amplification. The validity of the control strategy has been verified by the case study results.