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 redtide.github.io/elements/ .
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 index.md 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?