Spontaneous recurring appointments

I create an appointment in Outlook. Then I open the apt to edit details and when I save it often spontaneously converts to a recurring apt. I am very familiar with how to creat a recurring apt and how to turn this off, but how can I stop my Outlook from converting all my apts to recurring against my will? I am using Outlook 2007 and WinXP-Pro on a Dell Vostro 1500.