Unique values in tags

Hello all:

 After 3 months away from xsl development, i'm gladly returned to it,
and to this excelent, one-of-a-kind, superb list.

 I have a question and i don't know if the answer is XSL or Schema,
but here it goes.

 I have a XML file with an associated Schema. That XML file will be
edited on some XML editors (Oxygen and/or Exchanger) and i have to
force the user to only insert unique values on a certain tag,
something like a key.

 So the question is: Is it possible to define in the Schema that
certain tag has unique values?

 If not, is it possible using similar aproaches like DTD or RelaxNG?

 if not, is it possible using XSL/XSLT?

 Thanks a lot.

 One more question, is it ok to post Schema related questions on this
list? If not, can someone point me to one?

 Thanks again.