In this paper we present results of direct numerical simulations of lean hydrogen-air flames freely propagating in planar narrow adiabatic channels, using detailed chemistry and transport. Our simulations show that double solutions, symmetric and nonsymmetric, can coexist for the same set of parameters. The symmetric solutions were calculated imposing symmetric boundary conditions in the channel midplane and when this restriction was relaxed non-symmetric solutions developed. This indicates that the symmetric solutions are unstable to non-symmetric perturbations, as predicted before within the context of a thermo-diffusive model and simplified chemistry. Moreover, the propagation speeds of the symmetric and the non-symmetric flames are found to be very different, and therefore the flashback critical conditions also differ significantly. This analysis seems to have received no attention in the literature, even if it can have important safety implications in micro-scale combustion devices burning hydrogen in a lean premixed way. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.