  We present the observation of doubly-produced $J/\psi$ mesons with the D0
detector at Fermilab in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV. The
production cross section for both singly and doubly-produced $J/\psi$ mesons is
measured using a sample with an integrated luminosity of 8.1fb$^{-1}$. For the
first time, the double $J/\psi$ production cross section is separated into
contributions due to single and double parton scatterings.
  Using these measurements, we determine the effective cross section
$\sigma_{eff}$, a parameter characterizing an effective spatial area of the
parton-parton interactions and related to the parton spatial density inside the
nucleon.
