Copying items from one project to another saves you from adding the items more than once.

First, make sure you're in the Builder section of your project. Then, click on Edit Items to show the list of additional buttons.

Select the line items you'd like to copy, then click Copy to Project. In the overlay you can select which project you'd like to copy the item to. Once you have selected the project, click Next. 

You will then be asked to select the room that you want the item(s) to be copied to. Once you have selected a room, click Next. The item(s) will then be distributed accordingly.

NOTE ABOUT SYSTEMS: If the item(s) you selected to copy into a project are in a System that does not exist in the other project, that System will be created. If the item(s) you selected to copy into a project are in a System that does exist (meaning it matches the System name exactly), they will copy into that System of the project.