class Multi : public Print
{
  public:
  virtual size_t write(uint8_t c)
  {
    Serial.write(c);
    Serial.write(c); // halt ein telnet write blabla
    return 1; // assume success
  }
};

Multi multi;

void setup() {
  Serial.begin(115200);
  //und das Telnet.begin blabla
  multi.print(1);
  multi.print("a");
  multi.println("fein");
  multi.println(F("das auch"));
}

void loop() {
  // put your main code here, to run repeatedly:

}