To select multiple tasks in your list press and hold down the 'Ctrl' key, before selecting your chosen tasks (Mac users use the 'CMD' key).

As soon as you select more than one task the Assign, Complete, Copy and Delete buttons appear at the top of your task list. After you have selected the tasks, press the relevant button.

If you are copying the tasks, another popup will appear, select the workspace you would like to copy the task to.