This paper devises an endogenous growth model with physical capital accumulation, knowledge formation and R&D, in which technological progress has an erosion effect in human capital, and innovation is subject to externalities associated to the duplication of research effort. We provide conditions for the existence of a unique feasible steady-state equilibrium with positive long-run growth. For appropriate parameter values, the transitional dynamics of the model is represented by a two-dimensional stable manifold. Numerical simulations show that the incorporation of an erosion effect, as well as duplication externalities, significantly increases the ability of the model to fit the observed data.