  Just as string T-duality originates from transforming field equations into
Bianchi identities on the string worldsheet, so it has been suggested that
M-theory U-dualities originate from transforming field equations into Bianchi
identities on the membrane worldvolume. However, this encounters a problem
unless the target space has dimension $D = p + 1$. We identify the problem to
be the nonintegrability of the U-duality transformation assigned to the
pull-back map. Just as a double geometry renders manifest the $O(D,D)$ string
T-duality, here we show in the case of the M2-brane in $D = 3$ that a
generalised geometry renders manifest the $SL(3) \times SL(2)$ U-duality. In
the case of M2-brane in $D=4$, with and without extra target space coordinates,
we show that only the ${\rm GL}(4,R)\ltimes R^4$ subgroup of the expected
$SL(5,R)$ U-duality symmetry is realised.
