Filtering duplicate addresses in listen (ticket #2400). Due to the glibc bug[1], getaddrinfo("localhost") with AI_ADDRCONFIG on a typical host with glibc and without IPv6 returns two 127.0.0.1 addresses, and therefore "listen localhost:80;" used to result in "duplicate ... address and port pair" after 4f9b72a229c1. Fix is to explicitly filter out duplicate addresses returned during resolution of a name. [1] https://sourceware.org/bugzilla/show_bug.cgi?id=14969