private readonly Dictionary<PID, object> _processedMessages = new Dictionary<PID, object>();

  public Task ReceiveAsync(IContext context)
  {
      switch (context.Message)
      {
          case Credit msg when _processedMessages.ContainsKey(msg.ReplyTo):
              replyTo.Tell(_processedMessages[replyTo]);
              return Actor.Done;
          case Credit msg:
              _balance += amount;
              _processedMessages.Add(replyTo, new OK());
              replyTo.Tell(new OK());
              return Actor.Done;
          //...
      }
      return Actor.Done;
  }