@Override public final void enqueue(Callback<V> callback) {
      synchronized (this) {
        if (this.executed) throw new IllegalStateException("Already Executed");
        this.executed = true;
      }

      if (isCanceled()) {
        callback.onError(new IOException("Canceled"));
      } else {
        this.doEnqueue(callback);
      }
    }
Returns a copy of this object, so you can make an identical follow-up request.
zipkin/src/main/java/zipkin2/Call.java