Form data submit and store in databse and if email aready exist then show message

I am using this code to submit and store data in the database. I want if an email already exists then show a message like your email already exists. code:

<?PHP

    $con = mysqli_connect('localhost','rrrradadf','-rfsg?%DXFK#');

    // servername => localhost
    // username => -rfsg?%DXFK#'
    // password => -rfsg?%DXFK#'
    // database name => rrrradadf
    $conn = mysqli_connect("localhost", "rrrradadf", "-rfsg?%DXFK#'", "rrrradadf");

    // Check connection
    if($conn === false){
        die("ERROR: Could not connect. "
            . mysqli_connect_error());
    }

    // Taking all 5 values from the form data(input)
    $email = $_REQUEST['email'];

    // Performing insert query execution
    // here our table name is college
    $sql = "INSERT INTO emailtable VALUES ('','$email')";

    if(mysqli_query($conn, $sql)){
  header('location:index.html');

    } else{
        echo "ERROR: Hush! Sorry $sql. "
            . mysqli_error($conn);
    }

    // Close connection
    mysqli_close($conn);
    ?>