  Characterising materials at pressures of several megabar and temperatures of
a few thousand Kelvin is critical for the understanding of the Warm Dense
Matter regime and to improve planetary models as these conditions are typical
of planets' interiors. Today, laser-driven shock compression is the only
technique to achieve multimegabar pressures, but the associated temperatures
are too high to be representative of planetary states. Double-shock compression
represents an alternative to explore lower temperatures. Here we present a
method to create well-controlled double-shocked states and measure their
thermodynamic state and optical reflectivity using standard optical diagnostics
(Doppler velocimetry and optical pyrometry) in a laser-driven shock experiment.
This method, which does not require the support of hydrodynamical simulations,
is based on the application of generalised Rankine-Hugoniot relations together
with a self impedance mismatch technique. A validation experiment has been
performed at the LULI2000 facility (\'Ecole Polytechnique, France) on a sample
of $\alpha$-quartz. A temperature $60 \%$ lower than along the principal
Hugoniot has been obtained at 7.5 Mbar.
