Barchans are propagating crescentic dunes that form in arid regions where strong, unidirectional winds blow across a firm soil lightly covered with sand. Recently, solitary barchan dunes have been shown to be unstable towards sand flux variation either growing to form mega-dunes or shrinking and disappearing when perturbed from equilibrium. However, observations of large corridors of barchan dunes in the field suggest that these bedforms are stable over long periods of time. Since dunes' migration rates are inversely proportional to their size, barchans of different size must collide and these collisions may be crucial in maintaining the stability of dunes in nature. Here, we first explain the unstable behavior of solitary barchans and then illustrate, using down-scaled physical experiments, the qualitative dynamics of binary collisions. A stability analysis, inspired by these experiments, suggests that collisions may indeed regulate the size of barchans migrating in a corridor by redistributing sand from large dunes to smaller ones.