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I would like to create a hierarchy of pages:
Index page (I have one, grid pattern, just right look, 6 links to Sub-Index)
Sub-index page (I want these to look identical to Index page)
Posts (I have one, looks OK, individual products).
What plugin (or duffers guide) to create the Sub-index page as a template copied from Index I can use and insert....?
If your theme doesn't use a page builder which has the ability to save layouts you could use https://wordpress.org/plugins/duplicate-post/ to duplicate the page, rename and edit.
As Darren says, if your theme doesn't have the replicate functionality you can either use a plugin, or click on the text Tab and copy the html and paste that into the text tab on the new page.