  For the Standard Model extended with a real scalar singlet field, the
modification of the heavy Higgs signal due to interference with the continuum
background and the off-shell light Higgs contribution is studied for gg --> ZZ,
WW --> 4 lepton processes at the Large Hadron Collider. Interference effects
can range from O(10%) to O(1) effects for integrated cross sections. Despite a
strong cancellation between the heavy Higgs-continuum and the heavy Higgs-light
Higgs interference, the full interference is clearly non-negligible and
modifies the heavy Higgs line shape. A |M_VV - M_h2| < Gamma_h2 cut mitigates
interference effects to O(10%) or less. A public program that allows to
simulate the full interference is presented.
