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This happens rarely and i expect that it has something to do with a browser setting. Yet I'm not quite sure. Any ideas?
Are you only setting a cookie on page B or do you try to set a cookie for every page past B. For instance do you see someone who goes A to B to C get two cookies issued to them (one on B and one on C)?
Try to look at your logs for more than two pages. If someone is refusing cookies then you will see a new cookie issued to an IP for every page requested.
Another scenario is that you have multiple people coming through the same IP and what you think is one person is really two.
The ab cookie is set once (supposedly) only when they hit a page (version a or b) only when that cookie does not exist.
I don't think these are robots because i will see a visitor cookie that has hit multiple pages. So it is accepting cookies.