my php registration file creates too many entries in the database - php

Question

I have a file named reg.php that extends my registration.php for a login system.
The registration is connected with a mysqli database and I have a manual
increase in user id in php instead of autoincrement in mysqli because I could
not make this work.
When I hit the register button, my code is creating two entries in the db and
I dont know why.
<?php
if(isset($_POST["register_user"])){
header('location: login.php');
}
if(isset($_POST["username"])){
$username = mysqli_real_escape_string($db, $_POST['username']);
}
if(isset($_POST["email"])){
$email = mysqli_real_escape_string($db, $_POST['email']);
}
if(isset($_POST["password"])){
$password = mysqli_real_escape_string($db, $_POST['password']);
}
if(isset($_POST["passwordconfirm"])){
$passwordconfirm = mysqli_real_escape_string($db, $_POST['passwordconfirm']);
}
$idcount = mysqli_num_rows(mysqli_query($db, "SELECT id FROM user"));
$regquery = "INSERT INTO user (id, name, email, password) VALUES ('$idcount', '$username', '$email', '$password')";
mysqli_query($db, $regquery);
?>
all I want is one single entry.

Answer 1

There's nothing in the code, in your question, that could insert two rows in your database. My guess it that you call this script twice, probably because your form has two ways of submitting the same data:
The normal way, with the form action attribute.
Through an event, like onSubmit
You've got to stop the former, if the latter is used. Look at the example in the link for 2. It tells you how to stop the form from submitting as well:
// For this example, don't actually submit the form
event.preventDefault();
So if the onSubmit works, it stops the normal form submission, but if it doesn't work, the form is submitted the normal way.