  We prove the uniqueness of the group measure space Cartan subalgebra in
crossed products A \rtimes \Gamma covering certain cases where \Gamma is an
amalgamated free product over a non-amenable subgroup. In combination with
Kida's work we deduce that if \Sigma < SL(3,\Z) denotes the subgroup of
matrices g with g_31 = g_32 = 0, then any free ergodic probability measure
preserving action of \Gamma = SL(3,\Z) *_\Sigma SL(3,\Z) is stably
W*-superrigid. In the second part we settle a technical issue about the unitary
conjugacy of group measure space Cartan subalgebras.
