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Thanks for the reply. I'll check out the links you included. I thought it might be helpful to show an example of what got me thinking along these lines. This is a study about A/B testing of title tag changes. They don't say they accounted for position fluctuation; maybe they didn't, but it seems like you'd have to in order to have meaningful results. https://www.rankscience.com/coderwall-seo-split-test

Thanks for the info. My concern is that there are some instances where Google is splitting the traffic between two PDP urls. I guess it's good to know it is a problem others have. From what I've read and what you've said, I don't see a solution without eliminating cross-listing of PDP's between relevant categories, and doing that (I think) would make for a poor user shopping experience.

Thank you everyone for your responses. Sorry for the belated reply -- I didn't realize this question had garnered any responses. This is helpful information and thanks again.

Thanks!

Thank you!  That helps a lot.