Probably an issue with your router.
Using the newest firmware for our Netgear MR814v2, my bro and I can play on the same CS:S/DoD:S/etc server even though we have the same IP from the cable modem. Some servers give warnings that we are "ghosting" but appearantly those genius server admins haven't heard of home networks yet.
But, that newest firmware disconnects both of our ICQs constantly. Can't stay connected more than 10 minutes. So we need to switch back to a couple of versions older to have ICQ stay connected but then we can't play on the same server anymore. I haven't found a firmware version yet that lets ICQ stay connected but allows us to both play on the same server. So we take ICQ functionality since we rarely are playing the same game though it was fun to play Source games on the same server.
Conclusion: Home routers and their respective firmware writers suck. But try updating yours.