twenty seventeen customizer duplicates section content

Here's a thing that baffles me. Others have reported similar problems but none that match exactly, and being pretty new to WordPress, I find myself a bit at a loss.

Using the twentyseventeen theme I succeeded in getting a static homepage with four sections that are each drawn from pages with their own featured photo. Everything was working fine until I made some modification that I couldn't now identify, and now the content of the top section is duplicated where there should be the content for the 3rd section. The generated HTML begins thus:

article id="panel3" class="twentyseventeen-panel  post-488 page type-page status-publish has-post-thumbnail hentry"

        div class="panel-image" style="background-image: url(http://neolog.webfactional.com/fireandgrace/wp-content/uploads/2018/03/fieldBackScaledUp-2000x1200.png);"
            div class="panel-image-prop" style="padding-top: 60%"/div
        /div!-- .panel-image --

etc. The panel displays the right featured photo, and the custom sidebar that I built for that page displays correctly. Moreover, the panel's article tag has class post-488, which post has the correct content. But the content that's actually displayed in div class="entry-content" for the section is a duplicate of the top section's content. If I replace the panel3 section with some new page, I get the display, but I can't get the content of this particular page to display in the section.

So, I thought it might be a problem of corrupted content in the database. So I had a look at the post_content fields tables, and so far as I can tell, the relevant posts are correctly identified and have the correct content.

If the assembled wise here can offer any insight, I'd be grateful. The (not ready for prime time) website is to be found here for the moment: neolography.com/fireandgrace