A strong signal for double parton (DP) scattering is observed in a
16 pb^-1 sample of pbar p -> gamma/pi^0 + 3 jets + X data from the CDF
experiment at the Fermilab Tevatron.  In DP events, two separate hard
scatterings take place in a single pbar p collision.  We isolate a large
sample of data (\sim 14000 events) of which 53% are found to be DP.  The
process-independent parameter of double parton scattering, sigma_eff, is
obtained without reference to theoretical calculations by comparing observed
DP events to events with hard scatterings in separate pbar p collisions: The
result sigma_eff=(14.5 +\- 1.7^+1.7_ -2.3) mb represents a significant
improvement over previous measurements, and is used to constrain simple models
of parton spatial density.  The Feynman x dependence of sigma_eff is
investigated and none is apparent.  Further, no evidence is found for
kinematic correlations between the two scatterings in DP events.