Page.title shown even if not set in frontmatter on Github, fine locally

I’m facing a strange problem with a website on GH pages, a preview is at .
While locally works fine (at least on 2 different machines), the result of the homepage on GH appears with 2 H1 headers, so the page title is shown twice, though I haven’t added a page.title in the frontmatter in by purpose to show the one in the content.
The code in default.html layout should show the page.title only if present. How is it possible?