  Double-$\gamma$ vibrations in deformed nuclei are studied in the context of
the interacting boson model with special reference to their anharmonic
character. It is shown that large anharmonicities can be obtained with
interactions that are (at least) of three-body nature between the bosons. As an
example the $\gamma$ vibrations of the nucleus
$^{166}_{\phantom{0}68}$Er$^{\phantom{00}}_{98}$ are studied in detail.
