I want to override page_layout file product-full-width.xml
I was trying to override this in
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page_layout represents the full page, i mean you can declare your own full page without inheriting any master or root or other empty layout.
layout represents the part of the page mostly the html content. You can use this when you want your page to inherit the common contents from the root layout like header, footer etc.
For overright page layout please refere below link :