  The production cross-section of $J/\psi$ pairs is measured using a data
sample of $pp$ collisions collected by the LHCb experiment at a centre-of-mass
energy of $\sqrt{s} = 13 \,{\mathrm{TeV}}$, corresponding to an integrated
luminosity of $279 \pm 11 \,{\mathrm{pb^{-1}}}$. The measurement is performed
for $J/\psi$ mesons with a transverse momentum of less than $10
\,{\mathrm{GeV}}/c$ in the rapidity range $2.0<y<4.5$. The production
cross-section is measured to be $15.2 \pm 1.0 \pm 0.9 \,{\mathrm{nb}}$. The
first uncertainty is statistical, and the second is systematic. The
differential cross-sections as functions of several kinematic variables of the
$J/\psi$ pair are measured and compared to theoretical predictions.
