How can I duplicate a task?
How can I view completed items?
Using iPad app
Unfortunately, neither is possible on the iOS app right now. On the web, you could use the import/export feature to duplicate a task list as shown on this short video: https://www.youtube.com/watch?v=_Rl_CLGY_3o
We'll implement completed tasks view on iOS in future updates.
For now, as a workaround, you could actually try to use the web-app on your iPad. It's fully HTML5 compatible and should work just fine on touch-controlled interfaces. To access it without being redirected to the mobile version, please use this URL: http://todoist.com/m/forceRegular