How to prevent users from inserting duplicate emails to SQL Server from PHP application?

Question

The code verifies that the email exist but it doesn't prevent inserting duplicates in the table. I would like to insert if it doesn't exist, and redirect to thank_you.php - Code looks like this:

<?php
if (isset($_POST['submit'])) {
    $firstName = $_POST['firstName'];
    $lastName = $_POST['lastName'];
    $username = $_POST['username'];
    $password= password_hash($_POST['password'], PASSWORD_DEFAULT);
    $country = $_POST['country'];
    $city = $_POST['city'];
    $state = $_POST['state'];
    $phone = $_POST['phone'];

    include 'connect_sql.php';

        $sql = "INSERT INTO [Sonic].[dbo].[member] ([firstName],[lastName],[UserName],[Password],[Country],[City],[State],[Phone])
        VALUES ('$firstName', '$lastName', '$username', '$password', '$country', '$city', '$state', '$phone')";

        $email = "SELECT UserName FROM [Sonic].[dbo].[member] WHERE UserName='$username'";

        $row = sqlsrv_fetch_array( $email, SQLSRV_FETCH_ASSOC);

        $stmt = sqlsrv_query( $conn, $sql );

        if( $row === false) {                 
        echo "<div id='message'> Username $username already exist</div>"; 
        }           
        else {          
        header('Location: thank_you.php');
        }
     }
?>