  The Wiedemann-Franz (WF) law establishes a link between heat and charge
transport due to electrons in solids. The extent of its validity in presence of
inelastic scattering is a question raised in different contexts. We report on a
study of the electrical, $\sigma$, and thermal, $\kappa$, conductivities in
WP$_2$ single crystals. The WF holds at 2 K, but a downward deviation rapidly
emerges upon warming. At 13 K, there is an exceptionally large mismatch between
Lorenz number and the Sommerfeld value. We show that this is driven by a
fivefold discrepancy between the $T$-square prefactors of electrical and
thermal resistivities, both caused by electron-electron scattering. This
implies the existence of abundant small-scattering-angle collisions between
electrons, due to strong screening. By quantifying the relative frequency of
collisions conserving momentum flux, but degrading heat flux, we identify a
narrow temperature window where the hierarchy of scattering times may
correspond to the hydrodynamic regime.
