  The interplay between flavour symmetries connecting Bs-->KK decays with the
recently measured Bd--> K0 anti-K0 decay and QCD Factorisation opens new
strategies to describe the decays Bs--> K0 anti-K0 and Bs--> K+ K- in the SM
and in supersymmetry. A new relation, emerging from the sum-rule for the Bs-->
K0 anti-K0 decay mode, is presented offering a new way to determine the weak
mixing angle phi_s of the Bs system.
