But what does it do, then? The page I links states that it uses a hash table. Hash tables apply a hash function to the key. Hash functions map arbitrary input data onto data of a fixed size. It's inevitable that collisions will occur. ~~even if you use some sort of clever workaround in the case of collisions, eventually you use up all the available outputs.~~ (my bad)

I'm not claiming that it will silently break! I'd be very interested in exploring the internals a little more and finding out how hard it is to get a collision in various implementations and how they behave subsequently.

EDIT: I've read chasil's comment and agree that it must be storing raw keys in the array. I guess awk uses separate chaining or something to get around hash collisions.