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We report the discovery of two concentric Einstein rings around thegravitational lens SDSSJ0946+1006, as part of the Sloan Lens ACS Survey. Themain lens is at redshift zl=0.222, while the inner ring (1) is at zs1=0.609 andEinstein radius $Re_1=1.43\pm0.01"$. The wider image separation ($Re_2=2.07\pm0.02"$) of the outer ring (2) implies that it is at higher redshift. Itsdetection in the F814W filter implies zs2<6.9. The configuration can be welldescribed by a total density profile $\rho_{tot} ~ r^-g'$ with $g'=2.00\pm0.03$and velocity dispersion $\sigma_{SIE}=287\pm5\kms$. [...] We consider whetherthis configuration can be used to constrain cosmological parameters exploitingangular distance ratios entering the lens equations. Constraints forSDSSJ0946+1006, are uninteresting due to the sub-optimal lens and sourceredshifts. We then consider the perturbing effect of the mass associated withRing 1 building a double lens plane compound lens model. This introduces minorchanges to the mass of the main lens and allows to estimate the mass of Ring 1$(\sigma_{SIE,s1}=94\pm30\kms)$. We examine the prospects of doing cosmographywith a sample of 50 double lenses, expected from future space based surveyssuch as DUNE or JDEM. Taking full account of the model uncertainties, such asample could be used to measure $\Omega_m$ and $w$ with 10% accuracy, for aflat cosmology.

The Sloan Lens ACS Survey. VI: Discovery and analysis of a double Einstein ring