interject-async (unused) type signature contains duplicated effects?

[osa1]

https://github.com/koka-community/std/blob/f98bee331da917569a28c0cfb331d40484632275/std/async/async.kk#L511-L512

inline extern interject-async( action : () -> <async|e> a) : total ( () -> <async,async|e> a)
  inline "#1"

How can an effect type contain the same effect multiple times?

Also, this function seems to be unused. How is it supposed to be used?