You can duplicate an existing base that you've already created in Airtable by clicking the menu button on the base you wish to copy, and selecting "Duplicate base."
You will then be presented with a dialog that will allow you to choose the team in which you'd like to place the duplicated base.
You can choose whether you'd like to duplicate all the records in the base, or just the table, field, and view structure.
Note that if you duplicate a base, the new base will not have the revision history of the old base.