The concept of wave-particle duality in quantum mechanics is just a way of expressing the fact that many of the physical phenomena we observe (such as the interaction of light with matter and the propagation of electrons through a crystal) are not well predicted by intuitive classical models (eg of light as waves or of electrons as discrete particles), and that in some cases it looks more as if light is made up of discrete particles and electrons propagate as waves. Quantum mechanics is a mathematical model which always predicts what we do see in all situations, with the classical wave-like and particle-like behaviour being limiting cases predicted in exactly the situations that we do see them (which for light is usually more wave-like as the scale or strength of the signals is increased and more particle-like for small scale and/or very faint signals and for matter more the other way around).