Yesterday someone setup a Red Hat 6.0 box on our network and unfortunately assigned the same IP as our Red Hat 5.2 server. Needless to say both interfaces completely shut down, without out logging any errors or any notification. After much frustration we finally figured out what happened. If Windows 95 can tell you there is a duplicate IP out there I think Linux should as well.
I'm changing the component to kernel because that's where unices usually detect this problem.
You might also consider running arpwatch on a 3rd machine and monitoring the output. Arpwatch mails a message to root every time a new IP/MAC pair is detected. That might be sufficient for you to detect duplicated IP addr's but would require a 3rd machine and some sys admin work ...
Be very careful how you handle this. The Windows 95 "feature" is used regularly as a denial of service attack on windows networks by slamming the lan with faked addresses. It is possible to put it in the kernel but non trivial to do it right. If we need a patch for it I can knock one up
Resolved as "WONTFIX" because we aren't likely to fix this in the near future; there are no plans to fix it for the next release or next beta release or anything like that.