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The best solution is to do a 301 redirect to your preferred URL (likely the one without the index.html). That way people never see the index.html URL, never link to it, and all the juice goes to the other URL. To be able to give you instructions for this, it's best if you can tell us what type of server you are on (such as IIS or Apache), or if you can give us the URL we can figure it out from that. If you're not able to do a redirect, the canonical tag is the second best choice. Users will remain on the /index.html page, but at least Google then knows that it's the same page as /,