If a project using GPL-3.0-or-later doesn't include a copyright notice, does it violate the license? What would the consequences be?

I'm asking because <https://www.gnu.org/licenses/gpl-faq.html#IWantCredit> states:

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> [...] The GPL requires all copies to carry an appropriate copyright notice.
> 
> 
> 

Should the main `LICENSE` file in a repository already contain the copyright, or should copyright be handled on a file-to-file basis? (Most project *don't* do this.)