Why do people say 'mano y mano'? It's 'mano a mano.'

[Anaptyxis]

<Tick>
“Ah, chess! The ancient contest of wits. Two opponents, mano a mano. Braino a braino. And look! Magnets for ease of travel! You could play chess on the moon!”
</Tick>

I have nothing useful to add, except I’ve never heard “mano y mano”.