(CONS X NIL) = '(X), a list of length 1 containing X, so (CONS NIL NIL) = '(NIL), a list of length 1 containing NIL.
But NIL is a list of length 0, not a list of length 1.
The wart is that it should never have been the case that you could call CAR and CDR on NIL. But even though you can wartily call them on NIL, there is still clearly a very important distinction between NIL and (CONS (CAR NIL) (CDR NIL))!
Yes, that's true, but that is a special case. For all other objects X and Y, if (CAR X) is equal to (CAR Y) and (CDR X) is equal to (CDR Y) then X and Y are equal. NIL and (NIL) are the only exception. And the fact that they are an exception is a consequence of the design decision to allow CAR and CDR to be called on NIL and return NIL.
> The wart is that it should never have been the case that you could call CAR and CDR on NIL.
Yes, that is the whole point.
> But even though you can wartily call them on NIL, there is still clearly a very important distinction between NIL and (CONS (CAR NIL) (CDR NIL))!
You could have as well said between NIL and (CONS NIL NIL) or just (NIL). And yes, this is true. Nonetheless, it is possible to implement NIL as a privileged cons cell with both CAR and CDR pointing to itself under the hood, and many CL implementations actually do this. It's a design decision. You have to put the warty code somewhere. You can put it in CAR and CDR, or you can put it in NULL, EQUAL, SYMBOLP, etc. But you have to put it somewhere.
(CONS X NIL) = '(X), a list of length 1 containing X, so (CONS NIL NIL) = '(NIL), a list of length 1 containing NIL.
But NIL is a list of length 0, not a list of length 1.
The wart is that it should never have been the case that you could call CAR and CDR on NIL. But even though you can wartily call them on NIL, there is still clearly a very important distinction between NIL and (CONS (CAR NIL) (CDR NIL))!