Hey Hal,
How often do packets magically get duplicated within the network so that the target receives 2 copies? That seems like something somebody at NANOG might have studied and given a talk on.
I can't tell you how common it is, because that type of visibility is not easy to acquire, But I can explain at least one scenario when it occasionally happens. 1) Imagine a ring of L2 metro ethernet 2) Ring is connected to two PE routers, for redundancy 3) Customers are connected to ring ports and backhauled over VLAN to PE If there is very little traffic from Network=>Customer, the L2 metro forgets the MAC of customer subinterfaces (or VRRP) on the PE routers. Then when the client sends a packet to the Internet, the L2 floods it to all eligible ports, and it'll arrive to both PE routers, which will continue to forward it to the Internet. This requires an unfortunate (but typical) combination of ARP timeout and MAC timeout, so that sender still has ARP cache, while switch doesn't have MAC cache. In the opposite direction this same topology can cause loops, when PE routers still have a customer MAC in the ARP table, but L2 switch doesn't have the MAC. I wouldn't personally add code in applications to handle this case more gracefully. -- ++ytti