Neither Unity or Connection are designed for multiple users accessing the same mailbox at the same time - neither system "locks" mailboxes so you will always have windows for those types of issues (i.e. someone marks a message read right before another user goes to hear it). This is not a supported scenario in either product line and never has been.
Connection 7.0 does offer dispatch messaging, however, which is designed to provide a better solution for most uses that need "shared" mailboxes - you might want to look into it. In this setup all users have their own mailboxes (with their own notification device configuration etc...) and messages left for specific call handlers can be sent "dispatch" to all users defined in a public distribution list - everyone gets a copy, all their lamps are lit and/or notification devices fire off and all that good stuff. The first user to respond to a dispatch message has it removed from all other user's mailboxes. It's designed for scenarios like support groups where only one person needs to respond to a service call for instance.
