This depends on what you mean by "save a duplicate copy."
You can download (export) trees using the [Download / export this family...] botton (lower left of the tree screen)
If backing up your data is your goal you should export in GEDCOM or FamilyScript format. These files can be reuploaded to create a new tree if somehow your original gets damaged.
You can download and HTML copy of your tree that can be viewed offline.
To make a copy of your tree on Family Echo see this previous answer.