You can't have multiple attributes with the same name, that's a basic XML rule. Use child elements instead:
<FileHeader>
<idnumber>001</idnumber>
<idnumber>002</idnumber>
</FileHeader>
Alternatively it's sometimes appropriate to use a space-separated list of values
<FileHeader idnumber="001 002"/>
Michael Kay
http://www.saxonica.com/
Author, XSLT 2.0 and XPath 2.0 Programmer's Reference