Upon checking, I noticed that the duplicate H1 tag is applied to your store’s logo. You can view this here: screenshot.
To resolve this, I recommend changing the H1 tag to an H2 tag. Typically, the H1 tag for the logo can be found in the header.liquid file, though the location may vary depending on the theme.
Your first H1 is your logo image in header, it has no text and that’s why reported as “empty”.
This can be fixed by a simple theme code edit, but can’t suggest without seeing the code.
Sometimes themes have it as H1 on homepage only, but, say, H2 elsewhere – because other pages has main object (like product on product page, etc.). In your case it’s collection page, which is strange.
Also, you may reach out to the theme support – I’d consider it a bug they should fix.
Unless you have the latest version, they may say “upgrade to the latest” which may be difficult if you have code edits…
Update: Nope, their theme demo shop also has H1 logo on all pages.