AES-128: can any two keys generate the same ciphertext from the same plaintext?
So we already know that AES-128 has, in one context, the weird property that a series of values it generates is indistinguishable from a random series of 128-bit values, with the sole exception of "but you never get a duplicate until you've hit everything". I'm wondering if it might have that property in a different direction also.


Highly unlikely.  The weird property you point out was actually a design goal of AES (actually, of any block cipher), and is fairly easily seen if you open up the hood of AES and look inside - it's the concatenation of a number of fairly trivial permutations, and concatenating permutations together always gives you a permutation.

In contrast, it was never a design goal that two different 128 bit AES keys never map the same plaintext block to the same ciphertext block; as you point out:


  But with an actual series of 128-bit random values, you'd virtually certainly get at least one duplicate by the time you did all the inputs


So, unless AES-128 has some special properties that no one (even the designers) expect, there is almost certainly two keys that map one particular plaintext block to the same ciphertext block.