Sorry for the derail, Nik. Maybe this should be split into another thread. Nonetheless, I press on...
The rulebook says that it does not double. It explicitly says that. It plays the card twice, but does not double it.
Just checked the rule book and it says nothing about doubling or not doubling. TR doubles a card in that it results in one card getting played two times instead of one time.
But... it's not an abstraction. It's literally what happens. Throne Room makes you play the card twice, then you can no longer play the card.
Throne Room has you select a card, which you then play. Then TR has you play it again.
This is similar to but still distinct from what you described: TR has you select a card, which is never played. Then TR becomes
two full copies of another card. Note that you said that "the other card becomes unusable". That's what makes it an abstraction. That card does NOT become unusable. That card is played. But you are making an abstraction where "Throne Room is standing in for two full copies of the other card".
Another way to think about Throne Room is that it stands in for a second copy of the other card. Having TR-X in hand is very similar to having X-X in hand. It's not exactly the same, of course -- you can't play both X's if it's terminal, but you would get to play "both" with TR-X. Or if X is non-terminal, TR-X will leave you with an extra action(s) afterwards compared to playing X followed by X. So in this abstraction, you get an extra +1 action out of the TR itself.
Indeed, there is no reason to think of cards as cantrips that draw and play a card with the effect of the original card. I fail to see why Throne Room should be an exception.
But with TR, I'm not thinking of it as "a cantrip that draws and plays a card with the effect of the original card"! In this sentence, the original card is Throne Room. Rather, I am thinking of it as drawing and playing a card with the effect of whatever is played WITH Throne Room. That's why your statement with Militia is confusing, and it's the point of GeoLib's Stack Overflow joke.
And I'm not suddenly bringing TR into that example, the example has always been a comparison between Duchess and Throne Room, because they both have an effect that is pretty much "play an Abandoned Mine twice", if your Throne Room "target" is an Abandoned Mine.
That comparison still makes no sense. I explained (or at least tried to explain) how TR fits the riddle by referencing Throne Room and what the card effectively does.
Then you said that Duchess fits the original riddle just as well as Throne Room. So to show this, you'd have to explain how Duchess matches each of the clues only by referencing Duchess itself. You can't just randomly bring in Throne Room and Abandoned Mine into this explanation -- that's nonsensical!