Using the command-line utility 'adduser' you will avoid duplicate UIDs. In fact, even the lower-level 'useradd' should avoid it. Someone on your system may have used 'vipw' or otherwise manually edited /etc/passwd and associated files.

Thanks for the reply but my question is from a System Administrator's point of view. Securing (Hardening) Linux.

Is there a way to prevent duplicate IDs? A system level policy

How about an administrators' policy not to add users except using the tools that already implement it (and not to use useradd's override!)?

I'm wondering from what perspective you took my answer to be. Only administrators have access to those tools, and those tools, using their defaults, do indeed enforce that policy.

Ah, shouldn't have posted so soon. I guess what you're asking is "is there a way to prevent myself from shooting myself in the foot"?

Just off the top of my head, implementing /etc/passwd and /etc/shadow as symlinks to dynamic "files" implemented by a user filesystem (FUSE) could possibly work, at a tremendous cost in added complexity and possibility of subterfuge. Let's say you implemented it using some type of database, with the UID being a primary key; an administrator could remove the primary key attribute from the field, and you'd be back with the original problem.

Some problems are better solved by policy rather than technology. Of course, you could put /etc and anything else you want unchangeable on DVD or other write-once media, but then any time you wanted to change something you'd have to burn a new disk.

As stated, the system tools already prevent this. What is the problem?

To answer your question:
"is there any way or script why which duplicate UIDs can be prevented altogether."

The answer would be no.

Root can log in and edit passwd manually, and add a user with an existing UID.