It's now possible to duplicate tasks in Todoist on the web-based platforms (web app, Mac app and Windows legacy app). We will add it to mobile platforms soon.
It's easy to do:
1. Enter a project or a filtered view.
2. Right click a task you want to duplicate (or use the three-dot icon next to a task) and select Duplicate.
3. The task will be duplicated below with all its parameters (due date, priority, labels, sub-tasks).
* Please note that this feature currently doesn't copy any comments included in a task.