  A measurement of the branching fraction of the decay $B_s^0 \to K_S^0 K_S^0$
is performed using proton-proton collision data corresponding to an integrated
luminosity of 5 $fb^{-1}$ collected by the LHCb experiment between 2011 and
2016. The branching fraction is determined to be BR($B_s^0 \to K_S^0 K_S^0$) =
[8.3 $\pm$ 1.6 (stat) $\pm$ 0.9 (syst) $\pm$ 0.8 (norm) $\pm$ 0.3 ($f_s/f_d$)]
$\times 10^{-6}$ , where the first uncertainty is statistical, the second is
systematic, and the third and fourth are due to uncertainties on the branching
fraction of the normalization mode $B^0 \to \phi K_S^0$ and the ratio of
hadronization fractions $f_s/f_d$. This is the most precise measurement of this
branching fraction to date.
  Furthermore, a measurement of the branching fraction of the decay $B^0 \to
K_S^0 K_S^0$ is performed relative to that of the $B_s^0 \to K_S^0 K_S^0$
channel, and is found to be $\frac{BR(B^0 \to K_S^0 K_S^0)}{BR(B_s^0 \to K_S^0
K_S^0)}$ = [7.5 $\pm$ 3.1 (stat) $\pm$ 0.5 (syst) $\pm$ 0.3 ($f_s/f_d$)]
$\times 10^{-2}$.
