stay in the same value of analog input give output one time

Projects General Guidance
5

You just need to keep track of past reading through the main loop:

const int sensor = A0; // analog input use variable resistance
int flagged = 0;  

void setup ()
{
  Serial.begin (9600);
}

void loop ()
{
  if(analogRead(sensor)==1023)//when variable resistance give value =1023 print 'x' 
  { 
   if(!flagged) 
    {
      Serial.print("x");
      Serial.println();
      flagged = 1; 
    }
   else flagged =0;
  }
 
   delay (1000);
}