Vista submits 2 identical jobs to IPP print server with authentication

I've got a Windows 2000 print server that supports both standard print share
and IPP. For IPP, I turned on basic authentication and HTTPS so that users
require to login before they can submit print job.

The setup works fine for all Windows clients except Vista. I spot that on a
print server with authenticated IPP, Vista will submit 2 identical print
jobs. At server eventlog, I found that 2 entries of the submitted by Vista.
So users will get 2 copies for each submission. The problem does not occur
if the IPP print server has turned off authentication.

Does anyone here can give me some hints to fix the problem in Vista?

Thanks very much in advance.

Steve