  Dark matter (DM) could be a pseudo-Dirac thermal relic with a small mass
splitting that is coupled off-diagonally to a kinetically mixed dark photon.
This model, particularly in the sub-GeV mass range, is a key benchmark for
accelerator searches and direct detection experiments. Typically, the presence
of even a tiny fraction of pseudo-Dirac DM in the excited state around the time
of recombination would be excluded by DM annihilation bounds from the cosmic
microwave background (CMB); thus, viable thermal histories must typically
feature an exponential suppression of the excited state. We revisit assumptions
about the thermal history in the resonant regime, where the dark photon mass is
slightly more than twice the DM mass (to within $\sim10\%$), leading to an
$s$-channel resonance in the annihilation cross section. This resonance
substantially reduces the couplings required for achieving the observed relic
abundance, implying that in much of the parameter space, the DM kinetically
decouples from the Standard Model well before the final DM relic abundance is
achieved. We find that the excited state is not thermally depopulated in this
regime. In spite of this, we find that the presence of the excited state does
$\textit{not}$ violate CMB bounds, even for arbitrarily small mass splittings.
The present-day abundance of the excited state opens up the possibility of
signatures that are usually not relevant for pseudo-Dirac DM, including
indirect detection, direct detection, and self-interacting DM signatures.
