icmp encapsulation

Hi, I would like to ask&know how the GGSN holds and finds pdp contexts for icmp packets that have same source and destination address on the downlink? For example, if there are two hosts having same IP address behind two different network pinging same destination address and they are encapsulated at SGSN and then one GGSN receives both and saves the packet, decapsulates and sends to the destination. On the downlink destination responds to the two ping packets (they are different because they have different identifier in icmp packet), but then how does GGSN finds the pdp contexts again to encapsulate these packets and send back to the two SGSNs? The pdp context doesnt include thse icmp identifiers. There is no sockets either since icmp doesnt have sockets. On the downlink, when there are two icmp packets recieved by GGSN, there is only icmp identfier that distinguish each other, but this identifier is not in the pdp context. Would someone help me figure out this? Thank you all