  Ohm's law is one of the most central transport rules stating that the total
resistance of sequential single resistances is additive. While this rule is
most commonly applied to electronic circuits, it also applies to other
transport phenomena such as the flow of colloids or nanoparticles through
channels containing multiple obstacles, as long as these obstacles are
sufficiently far apart. Here we explore the breakdown of Ohm's law for fluids
of repulsive colloids driven over two energetic barriers in a microchannel,
using real-space microscopy experiments, particle-resolved simulations, and
dynamical density functional theory. If the barrier separation is comparable to
the particle correlation length, the resistance is highly non-additive, such
that the resistance added by the second barrier can be significantly higher or
lower than that of the first. Surprisingly, in some cases the second barrier
can even add a {\it negative} resistance, such that two identical barriers are
easier to cross than a single one. We explain this counterintuitive observation
in terms of the structuring of particles trapped between the barriers.
