A micromechanical model has been developed in order to capture the influence of a second population of voids on the coalescence of large primary voids. FE unit cell simulations have been performed by introducing the primary voids explicitly in a finite element mesh and by using a Gurson type model for the surrounding matrix in order to reproduce the influence of the second population. These simulations have guided the development of a closed-form void coalescence model. The new coalescence condition accounts for the softening introduced by the second population by integrating the Gurson model based on an approximate solution for the stress and strain field near the surface of the primary voids. The evolution of the primary voids is modelled using an advanced Gurson model involving an evolution law for the void shape. The model is applied to the prediction of the fracture strain of 6xxx aluminium alloys measured on smooth and notched round bars. The model successfully captures, without any parameter adjustment, the variation of the ductility as a function of the stress triaxiality for various shapes of the primary particles and various volume fraction of second population.