When two objects in the same compilation range have same named messages, their selectors get translated into two functions with the same name.

object First implements Object {
    (void) self |> "message" |> {}
}
object Second implements Object {
    (void) self |> "meesage" |> {}
}

./zircon First.zc Second.zc

(selectors produced):

void *zircon_message(void *self) {}
void *zircon_message(void *self) {}

hence the name collision.