javascript - How do i check for duplicate entries in a database using pdo, php and ajax?

i made this form in datatables with php and ajax in which i need to warn the user for trying to insert a duplicated entry, but for some reason when i have the check code datatables throws me the error "DataTables warning: table id=tablaProveedor - Invalid JSON response. For more information about this error, please see http://datatables.net/tn/1", but only when the if check exists, when i comment it the code works well, i don't know if is because i'm using a switch to make the crud work

The code i used for that case is

     case 8: //cliente

            $repe = $conexion->prepare("SELECT cedula FROM cliente WHERE cedula= '$cedula'");
            $repe->execute();
            if($repe->rowCount()>0){
                   echo '<script>alert("La cedula ya existe")</script>';
        break;  
                }else{
        $consulta = "INSERT INTO cliente (nombrec, tipo, cedula, telefono, direccion, ciudad) VALUES('$nombrec', '$tipo', '$cedula', '$telefono', '$direccion', '$ciudad') ";           
        $resultado = $conexion->prepare($consulta);
        $resultado->execute();      
        $consulta = "SELECT * FROM cliente ORDER BY id DESC LIMIT 1";
        $resultado = $conexion->prepare($consulta);
        $resultado->execute();
        $data=$resultado->fetchAll(PDO::FETCH_ASSOC);       
        break;    
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and the insert in ajax looks like

$(document).ready(function() {
    $('select').select2({
dropdownParent: $('#formProveedor')
});
var id_pro, opcion;
opcion = 10;
    
tablaProveedor = $('#tablaProveedor').DataTable({  
    "ajax":{            
        "url": "bd/crud.php", 
        "method": 'POST',
        "data":{opcion:opcion}, 
        "dataSrc":""
    },
    "columns":[
        {"data": "id"},
        {"data": "nombrec"},
        {"data": "tipo"},
        {"data": "cedula"},
        {"data": "telefono"},
        {"data": "direccion"},
        {"data": "ciudad"},
        {"defaultContent": "<div class='text-center'><div class='btn-group'><button class='btn btn-primary btn-sm btnEditar'><i class='material-icons'>edit</i></button></div></div>"}
    ]
});     

var fila;
$('#formProveedor').submit(function(e){                         
    e.preventDefault();
    nombrec = $.trim($('#nombrec').val());    
    tipo = $.trim($('#tipo').val());
    cedula = $.trim($('#cedula').val());    
    telefono = $.trim($('#telefono').val());
    direccion = $.trim($('#direccion').val());
    ciudad = $.trim($('#ciudad').val());                     
        $.ajax({
          url: "bd/crud.php",
          type: "POST",
          datatype:"json",    
          data:  {id_pro:id_pro, nombrec:nombrec, tipo:tipo, cedula:cedula, telefono:telefono, direccion:direccion, ciudad:ciudad ,opcion:opcion},    
          success: function(data) {
            tablaProveedor.ajax.reload(null, false);
           }
        });                 
    $('#modalCRUD').modal('hide');                                                  
});
        
 

$("#btnNuevo").click(function(){
    opcion = 8;          
    id_pro=null;
    $("#formProveedor").trigger("reset");
    $(".modal-header").css( "background-color", "#17a2b8");
    $(".modal-header").css( "color", "white" );
    $(".modal-title").text("A?�adir proveedor");
    $('#modalCRUD').modal('show');      
});
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the case 10 is just a select * from clientes.

everything works when i don't have that if doing something, i've been googling for days what to do but now i feel like my head is going to explode, please help

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