How to discard same string received at Arduino
Only print the string when he has changed?

String previousString = "";
String newString = "";

if(newString != previousString){
   println(newString);
   previousString = newString;
}


I'm not sure if it's applicable in your situation, just the first thing that came to my mind. You (kinda) should make a function of it, so that you don't have to copy/paste it all the time and it'll look way better.

EDIT: 

A fuction could be made like so:

String previousString = ""; //Put this above your setup code (global variables)

void NoSpamString(String input){ //Put this block before the setup
   if(input != previousString){
      println(input);
      previousString = input;
   }
}


You can then use: NoSpamString("text"); to print a line withouth spamming it.