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Session IDs, referral codes and other query strings
Exactly as explained in terms of canonicalization and consistency, having more than one URL that points to the same resource will cause issues. One character of difference to a URL, even if only in a query string appended to the end, or in capitalization, may be seen as an entirely different URL to a search engine.
Session IDs in particular make a site into a virtual bottomless pit to a search spider. It is designed to record and index each individual URL as its own entity, but a session ID in that URL will always lead to problems.
Session IDs cause problems for users too, and I have lost count of the number of times I have seen users pass URLs on to others via email, forums, etc. that contained a query string for a session that would expire before most ever found the link.
Avoid session IDs in URLs whenever possible, or at the very least, prevent them being indexed by search engines. The same treatment should be applied to query strings used to identify referrers, etc. Use a 301 redirect so that the spider is told not to index the URL with the query string (potentially attributing search traffic to some other campaign and throwing off all tracking attempts) and is instead given the primary (‘clean’) URL.
Look out for part 12 next week…