Before you customize a theme, it's a good idea to make a backup copy so that you can discard your changes and start again if you need to.
If you already have 20 themes added to your Shopify admin, then you'll need to delete one before you can duplicate another. Deleting a theme is a permanent action. You can download your theme before deleting it.
From your Shopify admin, go to Online Store > Themes.
For the theme that you want to duplicate, click the ... button to open the actions menu, and then click Duplicate.
The duplicate theme displays on the Themes page. It is named Copy Of + the name of the theme that you duplicated.