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I looked at the html source of your site. I think that this is an overly aggressive plugin or something that is trying to enforce what is essentially an unenforceable “rule”. The rule says that an html page should have only ONE H1 item.
This doesn’t really fit with most WordPress pages. In your case, there is an H1 for the site header, then a couple more in the content. If these are posts, then it would not be unusual for each post to have an H1 for the title.
I don’t know of any way to eliminate this. It really isn’t a bug, but a feature. It is really comes from how WP sites are typically generated. And in my opinion, it was a questionable “rule” to begin with. You can safely ignore the “warning”.