Higgs pair production at the LHC from gluon fusion is small in the Standard
Model, but can be enhanced in models where a resonant enhancement is allowed.
We examine the effect of a resonant contribution from a second scalar arising
in a model with a gauge singlet scalar field in addition to the usual SU(2)
scalar doublet, with mass up to $M_H$~600 GeV and discuss the interference
effects in double Higgs production. We compute the NLO QCD corrections in the
large $m_t$ limit and show that they can significantly distort kinematic
distributions near the resonance peak.
Comments: 27 pages, 22 figures. References updated, typos fixed, and discussion
added to introduction, Section IVB, and conclusion. Version to appear in PRD