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The canonical tag is useful but dangerous if not implemented correctly. I don't think it should be included in the core files. The best way for a store owner to address canonical issues if he does not fully understand the canonical tag, is to submit sitemaps. That way the SE will always know what the preferred URL is.
Anything can be dangerous if not implemented correctly, look at input fields, they too lead to holes in your script and hacking vulnerabilities.
While site maps are a good idea, they are not always followed strictly. If they were there would be no need to crawl the site at all. The Canonical tag contribution found here is a good start, with a few modifications it reduced duplicate content on a clients site from well over a 1000 to down to around 50.
This is a high ranked site and the duplicate content (from url issues) was effecting his placements a lot, once I implemented and modified the script, his placement returned.
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Force cookies to true will eliminate oscsid from url.
This will reduce the need of canonicl url.
My suggestion for a multistore shop is create an addiitonal flag stating to which store a product belongs as a product page that spider should crawl.
For the rest of site using this flag set nofollow noindex in meta tags.
SIDs are not creating the duplicate URL issue (generally SIDS are turned to false in the admin or should be so SE do not even see the SID) this issue comes from a URL like this : product_info.php?product_id=100&manufacturers_id=10 and product_info.php?product_id=100 being the same product and are noted with two links thus duplicate content.
Since the products are shared among many shops (that is the nature of the multi shop contribution), the flag would really not work well in a true multi shop system. If you are just using that for management of all products for many sites, then that is not the contribution / modification to use anyway.
The idea of mixing up the descriptions and other page info is going to be more efficient and with the canonical tags in place, should over all give better page results without the sharing of page rank for duplicate urls that really do not point to different products.
cheers
Edited by cannuck1964, 08 May 2011, 11:13.