Can't get it constant

[Steviey]

I read the docs on:
https://gogonzo.github.io/runner/articles/apply_any_r_function.html

... but I can't get it.

Is it possible to keep the window size constant while using an index?
For example this gives me varying window sizes:
do.call(runner,list(x=df,k=2,idx='id',f=doThatShit,simplify=FALSE,na_pad=FALSE))
while....
do.call(runner,list(x=df,k=2,f=doThatShit,simplify=FALSE,na_pad=FALSE))
gives me a constant window, but not in the right interval.
I would need an offset of 5 instead of 1 step forward, or exactly 5 days without spreading dates across windows. How would we do this?

[Steviey]

Could it be, that an index has to be unique?