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).
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