How to avoid duplicate form entry if someone submits same data

Then use following code snippet:

<style>#lp-pom-text-487{display:none;}</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
  <script>
    $("#field_name").focusout(function(){
		var enroll = $('#field_name').val();
        var username = "API username";
        var password = "";
        $.ajax({  
             type:"GET",  
             url:"https://api.unbounce.com/pages/page_id/leads/",
             crossDomain: true,
          	 dataType: 'json',
             headers: {
                "Content-Type":"application/vnd.unbounce.api.v0.4+json",
                "Authorization": "Basic " + btoa(username + ":" + password)
             },
             success: function (data) { 
               var existStatus = "not found";
               var result = $.map( data.leads, function( value, index ) {
                 var enrollNumberFromDB = value['form_data']['field_name'][0];
                 if( enrollNumberFromDB.toLowerCase() == enroll.toLowerCase()){
                   if(existStatus == "not found"){
                     existStatus = "found";
                   }
                 }
                });
               	if(existStatus == "found"){ 
                 	$("#lp-pom-form-450 .lp-pom-button").prop('disabled', true);
                 	$("#lp-pom-text-487").css('display', "block");
                    $("#lp-pom-text-487 p span").text("You have already applied. If you are selected, we will get in touch with you soon.");                       
                 }else{
                    $("#lp-pom-form-450 .lp-pom-button").prop('disabled', false);
                    $("#lp-pom-text-487").css('display', "none");
                 }
             },
             error: function (jqXHR, textStatus, errorThrown) { 
                $("#lp-pom-form-450 .lp-pom-button").prop('disabled', true);
             	$("#lp-pom-text-487").css('display', "block");
                $("#lp-pom-text-487 p span").text("Something went wrong. Please try again!");
             }
          });  
    });
</script>

Explanations:

1. We are calling Unbounce API

https://api.unbounce.com/pages/b87999b9-a1e9-45e6-9586-3ddf989301c4/leads/
Refer : https://developer.unbounce.com/

2. Use Above APU with the help of AJAX Call.

3. The ajax call will give you all the leads captureed on a particular page.Now you can compare this leads data with the input field value using array.map() method.

4. We are disabling the submit button if the input value from the field exists in the API result data.

Thanks