  The dileptonic decays of doubly-charged Higgs bosons H^{\pm\pm} are
investigated in the minimal type-II seesaw model with one Higgs triplet \Delta
and one heavy Majorana neutrino N_1 at the TeV scale. We show that the
branching ratios {\cal B}(H^{\pm\pm} \to l^\pm_\alpha l^\pm_\beta) depend not
only on the mass and mixing parameters of three light neutrinos \nu_i (for
i=1,2,3), but also on those of N_1. Assuming the mass of N_1 to lie in the
range 200 GeV--1 TeV, we figure out the generous interference bands for the
contributions of \nu_i and N_1 to {\cal B}(H^{\pm\pm} \to l^\pm_\alpha
l^\pm_\beta): \sqrt{|\sin\theta_{i4} \sin\theta_{j4}|} \sim 10^{-8}--10^{-5},
where \theta_{i4} and \theta_{j4} measure the strength of charged-current
interactions of N_1. We illustrate some salient features of the interference
bands by considering three typical mass patterns of \nu_i, and stress that it
is very difficult to distinguish the type-II seesaw model from the triplet
seesaw model in such a parameter region at the Large Hadron Collider.
