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It always seems to assign either the / page or the // page to the supplemental result index in every instance I have seen it.
Are these pages going to get a duplicate content penalty?
Is this a true canonical issue or am I seeing something else?
Any idea what would cause it? In the instances I have seen there is no coding error (two sites though not my own, the respective owners have checked them as thoroughly as possible). Could it be an external linking error with someone linking to the wrong uri?
I've been wondering about this too. This thread may help: [webmasterworld.com...]
Since that discussion, I've seen it happen again on one of our other sites where it never occured before. It's still annoying me and I have no idea why it's happening.
So far, it hasn't caused any problems in the SERPS for those sites, but I haven't noticed these pages with two slashes actually showing up when I do site:domain.com search.
Sorry I'm not much help! It's a mystery to me too. I couldn't find any inbound or internal links that could be causing the problem.