With $("#excist_supp").on("change".... and $('#excist_supp').change(function()... you are attaching the same event to the same element twice.
You can create a temporary array of all the used list values and create a function to check if the current value is exist in that array or not using Array.prototype.includes().
Try the following way:
$("#excist_supp").on("change", function() {
if(!checkValue()){
$(".indexDrop").hide();
var newText = $("#excist_supp option:selected").text();
var newValue = $("#excist_supp option:selected").val();
$("#letterTable tbody").append('<tr><td><input type="text" id="sm_supp" value="' + newText + '" class="form-control newStyle1" name="sm_supp" readonly></td><td><input type="text" id="sm_code" value="' + newValue + '" class="form-control newStyle2" name="sm_code" readonly></td><td><button type="button" class="adRow ibtnDel newExcist" style="width:30%;position: relative;right: 25%; margin-bottom:0px">x</button></a></td></tr>');
$("#letterTable tr").each(function(i) {
var count = (i) - 1;
if (i === 0) return;
var input1 = $(this).find('.newStyle1');
var input2 = $(this).find('.newStyle2');
// id
input1.eq(0).attr("id", "sm_supp" + count); // Text fields
input2.eq(0).attr("id", "sm_code" + count);
// name
input1.eq(0).attr("name", "sm_supp[" + count + "]"); // Text fields
input2.eq(0).attr("name", "sm_code[" + count + "]");
});
}
});
/* compare two fields */
var temp = [];
function checkValue() {
var listVal = $('#excist_supp').val();
var tableVal = $('.newStyle2').val();
var nawTab = JSON.stringify(tableVal);
if (temp.includes(listVal)) {
alert('Matching!');
return true;
} else {
alert('Not matching!');
temp.push(listVal)
return false;
}
}<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css">
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/js/bootstrap.min.js"></script>
<script src="https://code.jquery.com/jquery-3.4.1.slim.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/umd/popper.min.js"></script>
<div class="row">
<!-- Existing Suppliers -->
<div class="col-sm-6">
<div class="col-12">
<div class="row">
<div class="col-12">
<input class="form-control serch" id="supSearch" type="search" placeholder="Search Existing Suppliers..">
<div class="selector">
<select class="col-12 listbox newBox" name="List_0" size="20" id="excist_supp" style="height: 125px !important;">
<option class="indexDrop">Choose Existing Suppliers</option>
<option value="0000">Komal </option>
<option value="0001">Ranjeet</option>
<option value="0002">Vishal </option>
<option value="0003">Gaurav</option>
<option value="0004">Dhanpat</option>
<option value="0005">Joe</option>
<option value="0006">Gowri</option>
<option value="0007">shankar</option>
<option value="0008">Dhanpat</option>
</select>
</div>
</div>
</div>
</div>
</div>
<!-- Table -->
<div class="col-5">
<div class="container">
<div class="row clearfix">
<div class="table-wrapper">
<div class="table-scroll">
<table id="letterTable" class="table table-bordered table-striped order-scroll order-list" style="width: 95%;">
<thead>
<tr style="background-color: #680f79;color: #fff;">
<th class="text-center">Supplier Name</th>
<th class="text-center">Supplier Code</th>
<th class="text-center">Action</th>
</tr>
</thead>
<tbody id="mytbody" style="text-align: center;">
</tbody>
</table>
</div>
</div>
</div>
</div>
</div>
</div>