It doesn't have to be a marathon, or even much effort at all for something along these lines. Diagnostics>Packet capture, pick the affected WAN, filter on the IP you're pinging from, start the capture. Run the ping until you get duplicates and stop the capture. Then just a glance through the output will show echo request, echo reply, repeatedly. If you don't see more than one echo reply for each request, you know it's not coming from the firewall itself.
One being a wireless ISP, that's almost certainly where it's being duplicated. It's probably indicative of excessive buffering or another problem within their network, I would contact the ISP about it.