It is true that URL parameters, like session IDs or tracking Ids, can be the reason behind duplicate contents. This was revealed by Google itself. Google explained that, “when user and/or tracking information is stored through URL parameters, duplicate content can arise because the same page is accessible through numerous URLs.”

Even Adam Lasnik also supports this view point. He referred this in "Deftly Dealing with Duplicate Content".

When two identical content of two different URLs are crawled by search spider, there may be several negative effects, like:

• Multiple URLs can dilute link popularity
• Search results will show user-unfriendly URLs

To help users and webmasters with duplicate content, Google has designed a new algorithm. This new algorithm will:

• Detect duplicate content, and group the duplicate URLs into one cluster
• The "best" URL is selected to represent the cluster in search results
• Then the properties of the URLs are consolidated in the cluster to the representative URL

If one finds duplicate content in their site, they can you help search engines by:

• Removing unnecessary URL parameters
• Submitting a Sitemap with the canonical version of each URL.

To reduce duplicate content more broadly, Google came up with suggestions like:

1. When tracking visitor information, use 301 redirects to redirect URLs with parameters like affiliateID, trackingID, etc. to the canonical version.
2. Use a cookie to set the affiliateID and trackingID values.

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