Rather than immediately producing an additional specialized gene, genome duplication essentially allows for a "back up" gene. As there are now, theoretically, two functional versions of the gene, mutations which affect function in one of them can be compensated for by the other copy. This can allow for the evolutionary accumulation of mutations in one of these copies, allowing for a new function to be adopted over time while the other copy maintains the original function. An excellent example of this is the major histocompatibility complexes of the immune system. Humans have both MHC type I and II complexes, which are important in adaptive immunity. Invertebrates, however, only have one MHC complex. In this case, a duplicative event allowed for jawed vertebrates to adapt specialized functions for the two versions of the two MHC complex copies over time. In summary, specialization occurs following a duplicative event through evolutionary mechanisms which are no longer constrained by the necessity of maintaining function of a single copy of a gene key to survival.
