Impedances and V,I duality seems straightforward.  So what about the
lowpass/highpass duality of RC networks?

The difference is that R,C duality is about a different kind of
transfer function.  Impedances are current--to--voltage transfer
functions while for the R,C duality the transfer function is
voltage--to--voltage.

Writing it out for a lowpass and highpass network we get $T_l = 1 / (1
+ Z_R / Z_C)$ and $T_h = 1 / (1 + Z_C / Z_R)$.  The duality maps
$j\omega RC$ to its inverse.

The main difference is that non--impedance transfer functions require
a different operation: multiplication.  Imedances just need addition
and inversion, while a transfer function needs multiplication of
impedance and admittance, so it seems fundamentally different.

% [1] http://en.wikipedia.org/wiki/Duality_%28electrical_circuits%29