  Quantum materials with non-Kramers doublets are a fascinating venue to
realize multipolar hidden orders. Impurity probes which break point group
symmetries, such as implanted muons or substitutional impurities, split the
non-Kramers degeneracy and exhibit a Janus-faced influence in such systems:
they can destroy the very order they seek to probe. Here, we explore this
duality in cubic osmate double perovskites which are candidates for exotic
$d$-orbital octupolar order competing with quadrupolar states. Using {\it ab
initio} computations, Landau theory, and Monte Carlo simulations, we show that
Janus impurities induce local strain fields, nucleating quadrupolar puddles and
suppressing the octupolar $T_c$. At the same time, strains mix the non-Kramers
doublet with an excited magnetic triplet, creating parasitic dipole moments
which directly expose the hidden octupolar order parameter. Our work unravels
this Janus duality in recent impurity nuclear magnetic resonance (NMR)
experiments, with important implications for uncovering hidden order in diverse
multipolar materials.
