how to avoid duplicate data with php - php

Question

How do I avoid duplicate data when we enter the same data again? instead, I want to issue a warning:
sorry your username has been used
I'm using php script, I have done it to overcome duplicate data but the results have unexpected errors.
This is my code, please correct it if you wish. thanks
<?php
if (isset($_POST['submit'])) {
$npm_siswa = $_POST['npm_siswa'];
$nama_siswa = $_POST['nama_siswa'];
$bidangst_siswa = $_POST['bidangst_siswa'];
$ipk_siswa = $_POST['ipk_siswa'];
$query ="INSERT INTO class(npm_siswa, nama_siswa, bidangst_siswa, ipk_siswa) VALUES('$npm_siswa','$nama_siswa','$bidangst_siswa','$ipk_siswa')";
$insert_data = mysqli_query($con, $query);
$goto = header('location: input_data.php');
if (headers_sent($goto) == true) {
exit();
}
}
$checking = "SELECT * FROM class WHERE npm_siswa ='$npm_siswa'";
$process = mysqli_query($con, $checking) or die(mysqli_error());
while($row_filter = mysqli_fetch_assoc($process)) {
$kelasId = $row_filter['kelasId'];
$npm_siswa = $row_filter['npm_siswa'];
$nama_siswa = $row_filter['nama_siswa'];
$bidangst_siswa = $row_filter['bidangst_siswa'];
$ipk_siswa = $row_filter['ipk_siswa'];
}
if (mysqli_num_rows($process) > 0) {
echo "maaf nama anda sudah digunakan";
} else {
$data = "INSERT INTO class(npm_siswa, nama_siswa, bidangst_siswa, ipk_siswa) VALUES('$npm_siswa','$nama_siswa','$bidangst_siswa','$ipk_siswa')";
$check = mysqli_query($con, $data);
}
?>

Answer 1

The easy way is to first select rows from mysql using a select statement, and then check if the array is empty.
$checking = "SELECT * FROM class WHERE npm_siswa ='$npm_siswa'";
$process = mysqli_query($con, $checking) or die(mysqli_error());
if(empty($process)) {
echo "maaf nama anda sudah digunakan";
} else {
$data = "INSERT INTO class(npm_siswa, nama_siswa, bidangst_siswa, ipk_siswa) VALUES('$npm_siswa','$nama_siswa','$bidangst_siswa','$ipk_siswa')";
$check = mysqli_query($con, $data);
}

Answer 2

Another solution: make sure that it is the DBMS that controls the uniqueness, with a UNIQUE constraint. Then test your mysqli_query's return value to handle the error.
$process = mysqli_query($con, $checking);
if (!$process)
{
// Handle your error
}
Pros : Only one query
Cons : The error is not necessarily due to the UNIQUE constraint