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Coming back to your thread, one thing I see here is that you're using a rel=canonical pointing to a URL that points back to the original URL. Looking at your example:
.. contains <link rel="canonical" href="http://example.com/?topic=10"/>
... redirects back to A)
For our systems, this looks like a cyclic redirect, making it hard for us to follow and pick the right URL to actually index.
One way to resolve that could be to use the final URL as the preferred canonical one.