prevent duplicate xml entries using dtd

[TLKSCHP]

Here is my config.dtd:\[code\]<!ELEMENT tags (file*)><!ELEMENT file (tag*)><!ELEMENT tag EMPTY><!ATTLIST file name CDATA #REQUIRED><!ATTLIST tag name CDATA #REQUIRED> \[/code\]Here is a sample xml file:\[code\]<?xml version='1.0'?><!DOCTYPE tags SYSTEM 'config.dtd'><tags><file name='drawing.svg'> <tag name='tag1' /> <tag name='tag2' /></file><file name='new text document.txt'> <tag name='tag1' /> <tag name='tag2' /> <tag name='tag3' /></file></tags>\[/code\]If possible, I want to prevent duplicate tags from being added to a file element.Example:\[code\]<?xml version='1.0'?><!DOCTYPE tags SYSTEM 'config.dtd'><tags><file name='drawing.svg'> <tag name='tag1' /> <tag name='tag2' /> <tag name='tag2' /></file><file name='new text document.txt'> <tag name='tag1' /> <tag name='tag2' /> <tag name='tag3' /></file></tags>\[/code\]So, under the first file element, there is a duplicate\[code\] <tag name="tag2">\[/code\]I would like a parser to generate some sort of error if someone were to do this. Any ideas? Is there something else I should be using for this?