Exactly. Vincent’s points above cover it nicely. It’s also bad for user experience, e.g if you have a comment form, comments will be spread across the duplicates rather than consolidated on one page.
Essentially if you’re exporting your webflow site, it’s not worth trying to strip the .html extension. You should however redirect non www to www (or vice versa) and redirect /index.html to the homepage, otherwise duplicate content problems occur. I’m not sure if webflow hosted sites automatically redirect index.html to the homepage, but it is best practice to for SEO.
There’s a post here from @bartekkustra with some good advice for .htaccess optimisations