When building Linesheets and Custom Lists, you can clone previously built ones, so you don't have to start from scratch. Also, cloning a Custom List that you didn't create allows you to edit and send the list.
Linesheets
From the left navigation menu, go to Linesheets. Select the ellipse menu in the upper-right corner and choose to either Clone as Custom List or Clone as Linesheet.
You can also open a Linesheet, select the kebab menu in the upper-right corner, and then choose to either Clone as Custom List or Clone as Linesheet.
Custom lists
From the left navigation menu, go to Custom Lists. Use the ellipse menu to the right of a list to select Clone as Custom List.
You can also open a Custom List and select Clone as Custom List.
Note: If you didn't create the list, you'll see the following message in the upper-left corner: This is a shared custom list created by another user. You don't have permissions to make changes. To edit, duplicate the list.