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Hi Richard. For the first issue, does your site have over 10k pages? I ask because I believe the crawler has a 10k limit and if you exceed that number any issues on extra pages can be missed. Otherwise I would recommend inspecting your site in Google WMT. The idea behind any crawl reporting is to locate issues before Google finds them. Since you are just starting here, Google has likely already discovered any issues on your site. I have not found any broken links in WMT which the moz crawler missed so I trust the tool; however, I have not worked with IIS. For the duplicate page issue, you should ensure only one version of each page is accessible. Determine which URL version is preferred, then either eliminate the other version(s) or 301 redirect them to the primary URL version. In the example you offered, I would suggest http://www.cylc.org/ is the cleaner, preferred version of the URL.