In an XML Schema (XSD) I am writing, I need to define an attribute which can occur multiple times inside its parent element.
Just to clear it with an example : the p
You can't. Attribute names are unique per element.
If you need to have multiple bits of data under the same name, then the usual solutions are either a space separated list or child elements.
<event department="foo bar baz" />
or
<event>
<department>foo</department>
<department>bar</department>
<department>baz</department>
</event>