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Im not sure if this is the right category...
Ive developed my own affilliate program, where affiliates can forward to my store via a http request parameter (affiliateid=xyz)

Now im wondering, each affiliate will try to spread the link out as much as possible (besides advertising on their own site), so it will end up on forums, groups, etc.

What happens when the search engine index the links and start showing them on search result pages? Then the affiliates will be earning money for "free".

Whats the best way to combat this? Change their affiliate id every 3 months? Doesnt seem like a good idea as large websites then have to go thru the hassle of changing all their links.

input is appreciated. Thanks in advnace.