constraints - odd question

["Simon St.Laurent" <simonstl@simonstl.com>]

Suppose I want to create a preferences or properties file where the
names of the elements correspond to the names of the properties
involved.  I'm not concerned with validation in the sense that certain
elements (it's all elements) must be present in a certain order, but
there may be a different kind of validation needed.

I want to make certain that no elements have the same names as their
siblings.  Duplication of a property name will lead to overwriting, and
they want a simple way to warn readers that the duplication has occurred
before loading the properties file into their particular environment.

This isn't hard to do in Java, but Java isn't a natural or easy answer
for widespread distribution to newbies.  I'm looking for something more
like a DTD - an XSLT style sheet might be acceptable.

Any thoughts?  Attributes come by this property somewhat naturally, but
this case is about nested structures.

(The question is part my curiosity, part my interpretation of an
occasionally recurring reader question.)