If this happens within your To-Do List, it usually is a corruption of the To-Do List.
If this happens within your Tasks folder, it usually is a view setting.
Duplicates in the To-Do List
The To-Do List can be shown in the To-Do Bar, in your Tasks folder and under your calendar. When you see duplicates in one of those sections, you'll probably see them in the others as well. If this happens, you will have to reset the To-Do Bar. To do this, start Outlook with the /resettodobar switch;
Windows 8 - Press WIN and X together on the keyboard. Type in outlook.exe /resettodobar
Windows Vista and Windows 7 - Start > type: outlook.exe /resettodobar in the box.
Duplicates Created in Tasks Folder