You can perform this function from the Work items tab in the left navigation menu or from the task directly. For example, if the task you want to duplicate is attached to a control, navigate to the control, open the task, and then follow the steps below.
From the left menu, select Work items.
Select the Tasks tab.
Select the task you want to duplicate.
In the right panel, click the ... (More options) icon, and then click Duplicate task.