This is not double payment. It's just payment. If the Gold OA publishers did not exist, then there would be more closed access / subscription journals than there are now, and you would be paying more / higher subscription fees.
That's a guess.
You can't assess the potential for double-dipping based on the amount of Gold OA articles that are published. In fact, it's probably not helpful to even think consider double-dipping at all.Double-dipping refers only to hybrid Fools-Gold (subscriptions + Fools-Gold). I doubt there's much uptake yet, but it's certainly not true that it's not worth "considering": It's worth very closely monitoring.
For a hybrid journal, there is a simpler point of comparison - APC vs average subscription revenue for closed access articles. Regardless of where the journal is positioned in the market, you would expect these should be roughly equivalent, or slightly favouring the APC.I think you are very mistaken!The hybrid-Gold publisher sets the price per article for their hybrid-Gold APC articles, and you can be sure they set APCs to ensure that their total revenue does not shrink. Unless they keep adjusting either the subscription price or the APC to keep the Fools Gold APC from increasing their revenue, they are double-dipping (since the APCs are over and above the subscription income, which is uncancelled and uncancellable -- until we have universal Green OA).
If there were 100% Gold OA -- whether Fools Gold or Fair Gold -- I would not be bothering with any of this, because my goal, as a researcher, is to solve the research accessibility problem (which OA solves), not to solve the journal affordability problem (which Green OA would solve).
Or we could find out that despite all expectations, nothing much happens to nothing much happens to subscription costs (because primary links are still to the publisher's site, etc.) - in which case, nothing much changes.It is definitely a possibility (though I think an unlikely one) that universal mandatory Green OA will only solve the research accessibility problem, but not the journal affordability problem:
PS My pejorative term "Fools-Gold" obviously only refers to paid-Gold, not to that vast-majority of Gold OA journals that do not charge APCs at all, but subsist instead on subscriptions or subsidy. Alas those free-Gold journals are not among the must-have journals that this is all about. (Keep that in mind too.)