Realization: Is the WoL a primal?!

[Jandor]

That's sort of how it works, and was explained in an interview in the last year or so.

Basically the summoning of a specific primal based on a specific belief set uses a specific essence/aether to create that primal (though how the initial essence is chosen is, afaik, still a matter of theory). Once it has manifested as a primal, future summonings based on that belief will use that same essence (which is why Ramuh has memory of older days, for example), and thus a second identical version can't be summoned while the first remains.

Have you got a link? I think I must have missed that one and it sounds interesting.