JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://dsmorse.github.io/gridster.js/demos/assets/css/demo.css">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/jquery.gridster/0.5.6/jquery.gridster.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://dsmorse.github.io/gridster.js/dist/jquery.gridster.min.js"></script>
<div class="gridster">
    <ul>
    
    </ul>
        
</div>

	<button class="btn btn-outline-info" id="check"> Check If unique selected </button>

JavaScript

var gridster;

gridster = $(".gridster ul").gridster({
  widget_base_dimensions: [100, 100],
  widget_margins: [5, 5],
  helper: 'clone',
   serialize_params: function($w, wgd) {return {images: $w.find('textarea').val().trim() , col: wgd.col, row: wgd.row, size_x: wgd.size_x, size_y: wgd.size_y}},
  resize: {
    enabled: true
  }
}).data('gridster');

var json = [{
   
    "col": 1,
    "row": 1,
    "size_y": 2,
    "size_x": 2
}, {
   
    "col": 4,
    "row": 1,
    "size_y": 2,
    "size_x": 2
},

{
    
    "col": 6,
    "row": 1,
    "size_y": 2,
    "size_x": 2
},


{
    "col": 1,
    "row": 3,
    "size_y": 1,
    "size_x": 1
}, {
   
    "col": 4,
    "row": 3,
    "size_y": 1,
    "size_x": 1
},

{
    "col": 6,
    "row": 3,
    "size_y": 1,
    "size_x": 1
}

];


for(var index=0;index<json.length;index++) {
   
		

gridster.add_widget('<li class="new" ><button class="select-element" style="float: right;">-</button></li>',json[index].size_x,json[index].size_y,json[index].col,json[index].row);
}

var parentElement = [];

$(document).on("click", ".select-element", function() {
 
 		 var current = $(this).closest('li'),
         index = inArray(current, parentElement);
     if(index < 0) {
         current.attr("style", "background-color:red");
		 	   parentElement.push(current);
     } else {
         parentElement.splice(index, 1);
         current.removeAttr("style");
     }
		
     console.log("Parent Element Array Length");
		 for (var i = 0; i < parentElement.length; i++) {
		  		 console.log(parentElement[i]);
		}
			
});
	

$(document).on("click", "#check", function() {
	       
     alert(checkIfArrayIsUnique(parentElement));
		
	});
  

function inArray(element, myArray) {
		for (var i = 0; i < myArray.length; i++) 
        {
            if ($(myArray[i]).is($(element))) 
            {
               return i; // means there are duplicate values
            }
        }
        return -1;
}  
  
  
function checkIfArrayIsUnique(myArray) 
   ...