  Recently, flavour $SU(3)$ symmetry of strong interactions has been combined
with certain dynamical assumptions to derive triangle relations among $B$-meson
decay-amplitudes. We show that these relations allow a prediction of the
mixing-induced CP asymmetry $\acpmi(B_d\to K^0\bar K^0)$. Contrary to
statements made in several previous papers, this asymmetry should be
non-vanishing in the Standard Model due to QCD-penguins with internal up- and
charm-quark exchanges and could be as large as $\order(30\%)$. The branching
ratio BR$(B_d\to K^0\bar K^0)$ is expected to be of $\order(10^{-6})$. In the
future, the results presented in this letter should allow interesting tests of
the $SU(3)$ triangle relations and of the Standard Model description of CP
violation.
