[noise] Why no pattern omits the "ee" key exchange?

[Loup Vaillant David]

Hi,

I was just looking at the KK and IK patterns, and was wondering why the
respondent was performing both `ee` and `se`.  Couldn't they just do
`se`?  And it's not just KK and IK. *No* pattern omits `ee`.

Why?

I *guess* `ee` somehow prevents stuff like replay attacks or key reuse,
but I do not see how. Any idea what I'm missing?

Thanks,
Loup.