If you automatically serve an index.html when the url is /resource/ I presume you would also serve the same page at /resource/index.html which in practice means that you are again in same content at two different URLs land. I lean more toward the principle of be permissive in what you accept principle here and would serve the same content for: /resource /resource/ and /resource/index.html if presented with the url without doing a redirect. But in all my links or documentation standardize on just one of those. which in practice means that for most crawlers you'll only have one effective URL for the content, while still providing an experience that isn't annoying for users if they happen to type in a trailing slash for the browser.
The canonical link meta tag solves the SEO issue. Google uses this hint to help it understand which is the URL that you want to have indexed and avoids duplicate content. In this case you would add this meta tag:
