(Note: nscd is not available as a component when entering a bug, so, since component is mandatory, I've chosen yp-tools even though this doesn't have anything to do with that.) When multiple users with the same uid appear in the passwd file, mapping from uid to username always matches the first entry. This allows one to create multiple logins for the same uid -- a very useful feature, and one that has existed in UNIX environments for as long as I can recall. When using nscd, the semantics change -- the uid to username mapping resolves to whichever user was last accessed. For example, create a user called "dump" in the passwd file with uid 0. Make sure dump comes after root. Do ps aux. Most processes are owned by root. Now do su dump -fc whoami. Now do ps aux again. Most processes are owned by dump. whoami returns dump. If you stop nscd, this problem goes away. This can be a serious problem. A lot of programs don't work right if uid 0 doesn't map to root, but some software or configurations may require a particular username to be configured for a particular service. Other similar problems can occur with, for example, multiple logins to the same guest ftp account. Even though I think this is a fairly serious problem, I've suggested "normal" for both priority and severity since the workaround of not running nscd is simple and effective...
"Don't do that then". Having duplicate UIDs, you're on your own. Especially for root.