Echoing input command on serial reads

Getting an echo on the serial port
i send command "CMD1"
Micro should respond "SUC1"
But responce is
"CMD1
SUC1"

#define digitalPin2 2   // Port 2 for reading the door interlock state
#define digitalPin6 6   // Port 6 for Power LED on light field alignment
#define digitalPin8 8  // Port 8 for slecting dimming method
#define digitalPin10 10   // Port 10 for Green LED on filter
#define digitalPin12 12   // Port 12 for Green LED on filter
#define digitalPin13 13   //  Port 13 for reading the push button on the light field module 

int x =0;          
int interlockstate=0;
int pushbuttonlightfield=0;
int analogPin = A0;
//ISR//float value = 100;   //3035;


void setup()
{ 

  Serial.begin(9600); //begin serial data
   pinMode(digitalPin2, INPUT); // Port 2 for reading the interlock state
   pinMode(digitalPin6, OUTPUT); // Port 6 for Power LED on light field alignment is an output
   pinMode(digitalPin8, OUTPUT); // Port 8 for Dimming method select is an output
   pinMode(digitalPin10, OUTPUT); // Port 10 for Green LED on filter is an output
   pinMode(digitalPin12, OUTPUT); // Port 12 for Yellow LED on filter is an output
   pinMode(digitalPin13, INPUT); // Port 13 for reading the push button on the light field module 
   //pin 13 is not on an interrrupt for state change so might need to change the switch
}


void loop() 
{

      delay (500);    //rev 3, delay to slow system down

      //Read this to see if the push button to turn on the LED is pushed or not
       pushbuttonlightfield =  digitalRead(digitalPin13);
     //     Serial.print("Push button state (0 is not pushed): ");
    //      Serial.println(pushbuttonlightfield);

  //check if serial communication is avaiable
    //then issue responce if command recognized
    //using println rather than print so there is new line feed issued
    //took this part from Filter recogition code
  if (Serial.available() > 0) {
      String rx_byte = Serial.readStringUntil('\n');
        rx_byte.trim();
        
        Serial.println(rx_byte);
        if (rx_byte == "CMD1") {
            digitalWrite(digitalPin12, LOW); // turn off  Yellow LED in filter module
            Serial.println("SUC1");      //CMD1 responce
        }
        if (rx_byte == "CMD2") {
            digitalWrite(digitalPin12, HIGH); // turn on Yellow LED on filter module
          Serial.println("SUC2");      //CMD2 responce
        }
        if (rx_byte == "CMD3") {
          digitalWrite(digitalPin10, LOW); // turn on Green LED on filter module
          Serial.println("SUC3");      //CMD2 responce
        }
        if (rx_byte == "CMD4") {
          digitalWrite(digitalPin10, HIGH); // turn on Green LED on filter module
          Serial.println("SUC4");      //CMD2 responce
        } 
       if (rx_byte == "CMD5") {
          digitalWrite(digitalPin8, LOW); // Set Dimming method to Analog
          Serial.println("SUC5");      //CMD2 responce
        } 
       if (rx_byte == "CMD6") {
          digitalWrite(digitalPin8, HIGH); // Set Dimming method to Digital
          Serial.println("SUC6");      //CMD2 responce
        } 
       if (rx_byte == "CMD7") {
          digitalWrite(digitalPin6, HIGH); // turn on Power LED on light field alignment module
          Serial.println("SUC7");      //CMD2 responce
        } 
       if (rx_byte == "CMD8") {
          digitalWrite(digitalPin6, LOW); // turn off Power LED on light field alignment module
          Serial.println("SUC8");      //CMD2 responce
        } 
     if (rx_byte == "CMD9") {
          interlockstate  =   digitalRead(digitalPin2);  // 0 door open, 1 door closed
          Serial.println(interlockstate);  //(0 is door open)
        } 
 
  }

}

But responce is
"CMD1
SUC1"

String rx_byte = Serial.readStringUntil('\n');
        rx_byte.trim();
        
        Serial.println(rx_byte);

This first print statement is what you sent, that is "CMD1".

Then you get the conditional response

 if (rx_byte == "CMD1") {
            digitalWrite(digitalPin12, LOW); // turn off  Yellow LED in filter module
            Serial.println("SUC1");      //CMD1 responce
        }

Naming a String variable rx_byte is confusing and not good practice.

Working as coded.

OMG I looked over the code for a day and just kept skipping over that.
thank you