Why is my PHP / SQL generating duplicate database entries?

I'm quite new to PHP and an absolute beginner when it comes to SQL. I'm just learning the basics and I can't get my head around why my code is generating a duplicate entry every time the form is submitted, e.g.

Name: Joe Blogs Email: [email protected]
Name: Joe Blogs Email: [email protected]

The database has a table called user and two columns, name and email.

My index file looks like this, it has a simple form for name and email, and inserts the data on submit:

<form method="post" action="insert.php">
    <input name="name" type="text">
    <input name="email" type="email">
    <input type="submit" value="Submit Form">
</form>

<?php
$servername = "localhost";
$username = "DB_USER";
$password = "PASSWORD";
$dbname = "DB_NAME";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);

// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

$sqlout = "SELECT name, email FROM user";
$result = $conn->query($sqlout);

if ($result->num_rows > 0) {
    // output data of each row
    while($row = $result->fetch_assoc()) {
        echo "<b>Name:</b> " . $row["name"]. " <b>Email:</b> " . $row["email"]. "<br>";
    }
} else {
    echo "0 results";
}

$conn->close();
?>

<form method="post" action="wipe.php">
    <input type="submit" value="Wipe ALL Data">
</form>

This insert.php file is called when the form is submitted:

<?php
$servername = "localhost";
$username = "DB_USER";
$password = "PASSWORD";
$dbname = "DB_NAME";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);

// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

$sql = "INSERT INTO user ( name, email ) VALUES ( '{$conn->real_escape_string($_POST['name'])}', '{$conn->real_escape_string($_POST['email'])}' )";
$insert = $conn->query($sql);

if ($conn->query($sql) === TRUE) {
    echo "New record created successfully";
} else {
    echo "Error: " . $sql . "<br>" . $conn->error;
}

$conn->close();
?>

<a href="index.php">Back</a>

I've probably made some basic mistakes but I'm not sure why it is adding duplicates. Is it something to do with connecting twice to the database in each file? Is there a better way to connect only once? Or is it caused by the form submission itself?