A strong signal for Double Parton scattering (DP) is observed in
pbar p -> gamma + 3 jets data from the CDF experiment.  The
process-independent DP parameter, sigma_eff, is obtained using a new
technique.  The result, sigma_eff =(14.5 +/- 1.7^1.7_ -2.3)mb, represents a
significant improvement over previous measurements.  For the first time, the
Feynman z dependence of the sigma_eff parameter is investigated, and no
dependence is seen.