Notification of conflicting appointments

[NavyBuck]

My office has recently migrated from GroupWise to Outlook
2003. I notice that I can schedule different
appointments for the same date and time with no
notification from Outlook that there is a conflict. In
GroupWise, a pop-up would notify me of the conflict and
give me the option to go ahead and schedule the new
appointment or not. Is there a similar function in
Outlook that I could turn on?

Thanks