  We analyze a novel signature of the type II seesaw model - same-sign
tetra-lepton signal arising from the mixing of neutral Higgs bosons and their
subsequent decays to singly and doubly charged Higgs bosons. For this, we
consider wide ranges of the triplet vacuum expectation value (vev) and Yukawa
couplings, that are consistent with the observed neutrino masses and mixing as
well as the LHC search limits. We find that a doubly charged Higgs boson with
mass around 250 GeV and triplet vev around $10^{-4}-10^{-2}$ GeV can give
significantly large number of events through it decay to same-sign $W$ gauge
bosons at High-Luminosity LHC with $3000 \text{fb}^{-1}$ of data. We also
pursue the analysis for a future hadron collider with the c.m. energy of 100
TeV. Considering a heavy Higgs boson around 900 GeV and an intermediate region
of the triplet vev, where both same-sign dilepton and gauge boson decays can
occur, we identify a limited range of the parameters where the number of
same-sign tetra-lepton events are as large as 1000.
