Strange. I still think it is the Router's fault. It has a table of IPs it can distribute, vs the MAC address of machines it has granted IPs -- it shouldn't allow duplicates. If you've moved the 722's cable from one port to another on the switch that might have confused the switch, but the DHCP service should not have been fooled. Many home routers have a 30 day DHCP lease default, so maybe in the future one will go away. Anyway, you've done yourself proud to notice the anomaly, and learn some basic troubleshooting skills.
Doing the Google "Netgear wnr3500 duplicate IPs" there are a few obscure hits on the subject, but the links I get don't have search. So, you may not be the first to experience this.
BTW, on a simple switched LAN, communications is by MAC, not IP. When you ping an "unknown" IP, the ARP protocol sends an Ethernet packet to all devices on the network asking "who the heck has IP x.x.x.x? When someone responds, you have his return MAC and can communicate with him. This information is cached in your PC, displayed by "arp -a" or arp -g), but it only stays in the cache for 2 minutes or so. When troubleshooting, keep that in mind.