Customer is using Exchange 2010 SP2. Resource mailboxes are normal mailboxes with direct booking enabled. When Outlook 2007 users (prior to SP3) tries to book such a resource, the reject conflicts setting may not work. The could results in the same resource getting booked by different users in the same time slot.
This is a incompatibility issues with Outlook 2007 clients that have not been updated to SP3.
To avoid this issue, upgrade Outlook 2007 to SP3.
Article ID: 2970280 - Last Review: 21 May 2014 - Revision: 1