  In many new physics scenarios, the particle content of the Standard Model is
extended and the Higgs couplings are modified, sometimes without affecting
single Higgs production. We analyse two models with additional quarks. In these
models, we compute double Higgs production from gluon fusion exactly at
leading-order, and present analytical results in the heavy-quark mass ap-
proximation. The experimental bounds from precision electroweak measurements
and from the measured rate of single Higgs production combine to give
significant restrictions for the allowed deviation of the double Higgs
production rate from the Standard Model prediction as well as on the branching
ratio for the Higgs decay into photons. The two models analysed eventually
present a similar Higgs phenomenology as the Standard Model. We connect this
result to the magnitude of the dimension six operators contributing to the
gluon-fusion Higgs production.
