JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>JS Bin</title>
</head>
<body onload="duplicateCheck();">
  <textarea id="Serials" onkeyup="duplicateCheck()" rows="15">
   abcd   
a0.0\1*5  
b0015   
   54-JOF   
h h h
    
 dwm
abcd  
 ec

123
  </textarea>
  <div id="Output"></div>

<script>
    function duplicateCheck() {
    var output = document.getElementById('Output');
    	output.innerHTML = '';
    
    var textArea = document.getElementById('Serials'); 
    var inputSerials = 
        textArea.value.trim().split(/ *\n */);
	var outputSerials = new Object();
    
    for(var i = 0;i < inputSerials.length;i++){
    	var serial = inputSerials[i];
        
        //build an object whose properties are serials
        //if the serial exists, incremint a counter
        if(outputSerials[serial]){
            outputSerials[serial]++;
        }else{
            outputSerials[serial] = 1;
        }
    }    

   // For testing
   output.innerHTML = 
      'Serials: <br />'; 
    
   for(var i in outputSerials){
      output.innerHTML += i + " ";
   }
    
    output.innerHTML += 
       '<br /><br />Duplicate Serials: <br />'; 
   
    for(var i in outputSerials){
    //check to see if we have any duplicates
       if(outputSerials[i] > 1){
           output.innerHTML += i + " ";
       }
    }
}
</script>
</body>
</html>