You receive no warnings when a meeting is overwritten in your Outlook 2003 Calendar

When you open your Calendar in Outlook 2003, you may notice that one of your meetings has been replaced with another meeting. No warnings or messages appear to inform you the meeting has been replaced.

When you receive an instance of a meeting and then receive a series containing that meeting, the instance is deleted and only the series remains in your Calendar. When you receive two instances of the same meeting, the first instance received is deleted and only the most recently received instance remains in your Calendar.

This behavior is by design.