MS Exchange 2003; Outlook 2007 and 2003 - all SPs installed

I have set up a Resource for scheduling meeting room bookings.

When a second booking is made for the resource, there appears to be
inconsistency though between our users with most correctly receiving the
message "<resource> is already booked for the specified time.  You must use
another time or find another resource."

However, one user using Outlook 2003 is able to successfully make the second
(and third and...) bookings at the same time slot!  Other 2003 users are
getting the correct decline msg.

Why would this be different for that user?

Tks,
Jane