You can create a new template based on an existing template easily.
The videos and examples in this document are for "pages" template. You can do the similar steps for some type of template such as: products, collections, blogs, articles template.
Please follow step by step in this guideline.
1. Create a template in Theme Customize
Go to Theme customize > choose template > Create template.
You can enter any name for the template, example: "promotion". Then base it on the template you want to duplicate, example "about us".
Please follow the video below:
2. Assign Shopify data for template
Note: You need to Publish the theme first to see the template in Shopify database.
Go to Shopify > Pages > create or choose a page you need to have that new template > choose template you created in step 1.
Then you can customize the layout for this new template. It will not affect other templates.
Kindly follow this video:
3. Remove template
When you don't need the custom template, you can remove it. Go to Theme > Edit code > search for the 'template name'.json > click delete button. Please follow this video: