  Processes such as double Drell-Yan and same-sign WW production have
contributions from double parton scattering, which are not well-defined because
of a delta(z_\perp=0) singularity that is generated by QCD evolution. We study
the single and double parton contributions to these processes, and show how to
handle the singularity using factorization and operator renormalization. We
compute the QCD evolution of double parton distribution functions (PDFs) due to
mixing with single PDFs. The modified evolution of dPDFs at z_\perp=0,
including generalized dPDFs for the non-forward case, is given in the appendix.
We include a brief discussion of the experimental interpretation of dPDFs and
how they can probe flavor, spin and color correlations of partons in hadrons.
