  We study the transient heat current out of a confined electron system into a
weakly coupled electrode in response to a voltage switch. We show that the
decay of the Coulomb interaction energy for this repulsive system exhibits
signatures of electron-electron attraction, and is governed by an
interaction-independent rate. This can only be understood from a general
duality that relates the non-unitary evolution of a quantum system to that of a
dual model with inverted energies. Deriving from the fermion-parity
superselection postulate, this duality applies to a large class of open
systems.
