Surely this is a non issue. If an image is available on an RM site, and the same image on istock (but no other RF site) at a significantly lower price, why should istock worry? The RM site might though, as someone has already mentioned.

You said it, this would be an issue for the RM site. From Sean's website: Typically, for an image that has always been licensed as Rights Managed, the licensor is able to provide a history of usage to a prospective licensee.

How can you sell an image RM and provide history, when the same photo is up for sale as RF without any tracking?

Here's another way to look at it. If a photo has ever been licensed as RF it can never be sold RM. If a photo is up on IS it can't be sold anywhere else RF.

Since this thread has taken various directions beyond the original question, selling similar images from the same shoot is a different question. I don't see why not as they are different photos. "girl with glass of wine facing left" RF on IS "girl with glass of wine, facing right" RM Alamy.  Same setting, same clothes, same girl, but a different photo. Am I wrong about this?

If it's wrong, how different does it need to be to be "different"? Sounds complicated?

Yes, if I understand this right, you can be an exclusive at IS and sell RM images elsewhere. You just can't sell RF on other sites.