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Given the http://www.amazon.com/gp/product/1591845297/?tag=referly-20 URL how does referly know which of its users to pass Amazon's commission onto? Several could turn up to refer on the same product.

Probably via the "-20"

I think the "-20" is on every Amazon affiliate link.

No, Amazon dishes out -20 to all amazon.com IDs. Other countries have other numbers, e.g. -21 is amazon.co.uk.