  A strong-to-weak-coupling duality is established for the nonequilibrium
interacting resonant-level model, describing tunneling through a single
spinless level, capacitively coupled to two leads by a contact interaction. For
large capacitive coupling, U, and in the presence of a finite voltage bias, the
model maps onto an equivalent model with a small capacitive coupling \rho U'
\sim 1/ \rho U. Here \rho is the conduction-electron density of states. This
duality is generic to all high-energy cutoff schemes, however its details may
vary from one regularization scheme to another. In particular, it has the
status of an exact mapping within Abelian bosonization, applicable to all
repulsive interactions whether weak, intermediate or strong. On a lattice the
mapping is restricted to large U, and is controlled by the small parameter
1/\rho U. Explicit expressions are given for the low-energy scale, differential
conductance, and occupancy of the level in the limit where U is large.
