Am using an inhouse search engine which handles the canonical link tags and, therefore, doesn't index multiple, identical products. However it would appear that OpenCart may not be using this tag exactly as presented in RFC5988.
For example, RFC5988 defines a canonical like
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http://www.example.com/page.php?item=purse; rel="canonical"
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<link href="http://wightbooks.eu/walt-Disney-Snow-White-animation" ; rel="canonical" />
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<link rel="canonical" href="http://www.example.com/page.php?item=purse">;