duplicate schemalocation keys

We recently saw an instance document that supplied multiple schema location
values for the same namespace.  

The document starts:

<p1:envelope
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
  xmlns:p1="http://www.example.org/schemas/ns1" 
  xsi:schemaLocation="
http://www.example.org/schemas/ns1 Envelope.xsd 
http://www.example.org/schemas/ns1 Transaction.xsd 
http://www.example.org/schemas/ns1 Document.xsd 
http://www.example.org/schemas/ns1 Event.xsd">

I know the REC doesn't say what to do, but is there a common practice of how
to handle such locations?  Use first that exists? Combine all that exist?
Ignore all but first?  Give warning?

xan

Xan Gregg
TIBCO Software, Inc.
www.tibco.com