To duplicate a base, tap the dropdown arrow next to the name of the base. This will bring you to the Edit base menu.
From the Edit base menu screen, tap the Duplicate base option.
You will be given the option to copy all of the records in the base to your new duplicate base, or to create an empty base with the same field and table structure.
Then, you'll be asked to pick the team into which you'd like to put your duplicate base.