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We have a secure version of our site, as we often gather sensitive business information from our clients. Our https pages have been indexed as well as our http version. Could it still be a problem to have an http and an https version of our site indexed by Google?   Is this seen as being a duplicate site? If so can this be resolved with a rel=canonical tag pointing to the http version? Thanks

I'm relatively new to the technical side of SEO and have been trying to brush up my skills by going through Google's online Web-master Academy, which suggests that you need a If-Modified-Since HTTP Header tag on your site.  I checked and apparently our web server doesn't support this. I've been told by a good colleague that the If-Modified-Since tag is no longer relevant as the spiders will frequently revisit a site as long as you regularly update and refresh the content (which we do). However our site doesn't seem to of been reindexed for a while as the cached version's are still showing the pages from over a month ago. So two question really - is the If-Modified-Since HTTP Header still relevant and should I make sure this is included? And is there anything else I should be doing to make sure the spiders crawl our pages? (apart from keeping them nice, fresh and useful)