  We explore the dependence of the double ionization of the He atom on the
frequency of a strong laser field while keeping the ponderomotive energy
constant. As we increase the frequency we find that the remarkable
"finger-like" structure for high momenta recently found for $\omega=0.055$ a.u.
\cite{Staudte, Rudenko} persists for higher frequencies. At the same time, at
$\omega=0.187$ a.u. a new X-shape structure emerges for small momenta that
prevails in the correlated momenta distribution. The role of this structure as
a signature of the frequency dependence of non-sequential double ionization is
discussed.
