Preventing duplicate sources in a network that uses network address port translation on an established connection. In response to receiving an inbound packet at a destination host, input values are obtained therefrom and used to consult a mapping. If no match is found, a translation is performed, whereby a determination is made as to whether or not a port number is available within a range of port numbers that comply with a security association governing the connection. If so, an available port number is assigned to the connection, thereby avoiding a possibility of a duplicate source. If a port number is not available, the packet is rejected.