Prevent SoftSerial returning previously sent characters

If I'm sending characters using SoftSerial, these are returned together with the response from the 'remote station'. I'm using a SIM900 module with Uno; however the SIM900 does not return the received characters if I'm contacting it with (hardware) Serial.

Is this normal? If yes, is there a way to prevent it? Any help welcome!

Example code:

#include <SoftwareSerial.h>
SoftwareSerial SIM900_Serial(7,8);
String fromSIM = ""  ; // holds the string returned by the SIM900 module

void setup()  
{
  SIM900_Serial.begin(19200);            // software serial to SIM900
  Serial.begin(19200);                   // hardware serial ATmega to USB
  delay(5000);                           // wait for the serial communication to establish   

  SIM900_Serial.println("AT+CMGF=1");    // send AT command to set SIM900 to text mode
  delay(1000);                           // give the module some time to respond
  
  while (SIM900_Serial.available() > 0)
  { char inChar = (char)SIM900_Serial.read(); 
    fromSIM += inChar;
    delay(10);
  }
 
  Serial.println(fromSIM);  // prints "AT+CMGF=1 \n OK", but response from SIM900 is just "\n OK"

 }

void loop()
{
}