To duplicate / copy a project and it's tasks click on the Info icon next to the project title either on the dashboard or on your project board.
Now click on the ellipsis at the top right of the project dialog and select Duplicate...
You are then able to set a title and description for your duplicated project and to select which properties you want to duplicate:
Click on Continue to duplicate the project.
This way, you can use any project you've set up as a template, and reuse its structure over and over again.