src/main/java/io/reactivex/internal/observers/BasicFuseableObserver.java
@Override
    public void onError(Throwable t) {
        if (done) {
            RxJavaPlugins.onError(t);
            return;
        }
        done = true;
        downstream.onError(t);
    }
@Override
    public void onError(Throwable t) {
        if (done) {
            RxJavaPlugins.onError(t);
            return;
        }
        done = true;
        downstream.onError(t);
    }
-----------------------------------