DUPLICATION OPTIONS. “Following either tandem or segmental duplication events, one of the duplicated genes can be rendered non-functional via the accumulation of deleterious mutations. Alternatively, both duplicated genes can be maintained if the presence of both copies is advantageous. Over time, the function of these duplicated genes can diverge from the ancestral gene or from each other.”
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0001838