XML对DTD无效（相同的ID）

Question

So I've written this xml document but I fail to write an DTD in which it is valid to. I know that the problem is that the ID (Student name "DB") occurs twice in the document and therefor it is not uniqe. But how can I change my DTD to allow the ID to do that? I'm really green in this area so this might be a stupid question.

<?xml version="1.0" standalone="no"?>
<!DOCTYPE COURSES [
<!ELEMENT COURSES (COURSE*)>
<!ELEMENT COURSE (PROFESSOR+, STUDENT+)>
<!ATTLIST COURSE name ID #REQUIRED>
<!ELEMENT PROFESSOR EMPTY>
<!ATTLIST PROFESSOR name ID #REQUIRED>
<!ELEMENT STUDENT (NOTES*)>
<!ATTLIST STUDENT name ID #REQUIRED>
<!ATTLIST STUDENT enrolled IDREFS #IMPLIED>
<!ELEMENT NOTES (#PCDATA)>
]>
<COURSES>
<COURSE name="Architecture">
<PROFESSOR name="Andersson"/>
<STUDENT name="JB" enrolled="Architecture">
<NOTES>2nd BaSC</NOTES>
</STUDENT>
<STUDENT name="DB" enrolled="Architecture"/>
</COURSE>
<COURSE name="Mechanics">
<PROFESSOR name="Geerts"/>
<STUDENT name="DB" enrolled="Mechanics"/>
</COURSE>
</COURSES>

Answer 1

Use this link for validate your dtd/xml http://validator.w3.org/check , this website is the oficcial organization for standard this lengage.

<STUDENT name="DB" enrolled="Architecture"/>

Is replied.