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
When duplicate entries are created in the Tasks folder, it is most likely caused by a view setting. For Example: If you have a single task assigned to two categories, and you sort the tasks grouped by categories, then that task will show up in both categories. If you sort your tasks by Subject, they will only show up once.