  The excitation of two particle-two hole final states in neutrino-nucleus
scattering, not taken into account in Monte Carlo simulations for data
analysis, has been advocated by many authors as the source of the excess cross
section observed by the MiniBooNE collaboration in the quasi elastic channel.
We analyse the mechanisms leading to the appearance of two particle-two hole
states, and show that interference between the amplitudes involving one- and
two-nucleon currents, not consistently included in existing calculations, plays
a major role. A novel approach allowing to treat one- and two-nucleon current
contributions on the same footing is outlined.
