If Outlook displays duplicate reminders for the same event, and you don't actually have two appointments with the same content, or the same PST/OST in your profile multiple, you probably have a corrupt reminder "folder", an internal hidden folder containing your reminders.

You can rebuild the reminder folder by exiting Outlook completely, then starting it with a parameter:

outlook /cleanreminders

You can start Outlook with this parameter through the Run dialog (Start → Run, or WinKey + R).