  The observation of Higgs pair production at high-energy colliders can give
evidence for the presence of a triple Higgs coupling. However, the actual
determination of the value of this coupling is more difficult. In the context
of general models for new physics, double Higgs production processes can
receive contributions from many possible beyond-Standard-Model effects. This
dependence must be understood if one is to make a definite statement about the
deviation of the Higgs field potential from the Standard Model. In this paper,
we study the extraction of the triple Higgs coupling from the process
$e^+e^-\to Zhh$. We show that, by combining the measurement of this process
with other measurements available at a 500 GeV $e^+e^-$ collider, it is
possible to quote model-independent limits on the Effective Field Theory
parameter c6 that parametrizes modifications of the Higgs potential. We present
precise error estimates based on the anticipated ILC physics program, studied
with full simulation. Our analysis also gives new insight into the
model-independent extraction of the Higgs boson coupling constants and total
width from $e^+e^-$ data.
