This is a known and long-standing bug that actually pre-dates the forum. I’m going to close this report in favor of the first report of it on the forum: https://forum.inaturalist.org/t/observations-displayed-multiple-times-when-scrolling-in-grid-view/504
It will probably be fixed when the Explore page is rewritten, and for now, you can often fix it temporarily by refreshing the page.