PHP MYSQL Prevent user or email inserted twice - php

Question

I want to prevent if the user enters same email twice, which will create duplicate entry. Please help me to solve this small problem, I have searched stackoverflow for this problem but everyone has their own database with own method.
Thank you
<form action="create_subscriber.php" method="post">
<input placeholder="Name" name="inputName" type="name" required/>
<input name="inputEmail" placeholder="example#email.com" name="Submit" type="email" required/>
<input name="Submit" type="submit" value="Subscribe"/>
</form>
create_subscriber.php below
<?php
//include 'connection.php'
$dbhost = "localhost";
$dbuser = "sampleuser";
$dbpass = "samplepass";
$dbname = "sampledb";
$conn = mysql_connect($dbhost,$dbuser,$dbpass);
mysql_select_db($dbname);
$name = $_POST['inputName'];
$email = $_POST['inputEmail'];
if(!$_POST['Submit']){
echo "Please enter a name & email";
//header('Location: index.php');
}else{
mysql_query("INSERT INTO subscriptions
(`ID`,`Name`, `Email`) VALUES(NULL,'$name', '$email' ) ")
or die(mysql_error());
echo "User has been added";
header('Location: success.php');
}
?>

Answer 1

Since you are not doing any validation, you can use the Email as a unique field and do an REPLACE query. http://dev.mysql.com/doc/refman/5.0/en/replace.html
I would strongly advise you to write validation in the form of a query check against the database prior to attempting to do a secondary insert. It's even made easy by persistent connections being available so you don't have the overhead of few ticks it takes to do that validation query.

Answer 2

mysql_query("INSERT INTO subscriptions
(`ID`,`Name`, `Email`) VALUES (NULL,'$name', '$email' )
WHERE NOT EXISTS (
SELECT email FROM subscriptions WHERE email='$email'
)")

Answer 3

Simply execute a preliminary query to check for email uniqueness
SELECT * FROM subscriptions WHERE `email` = ?
(I wrote the query in the form of a prepared statement
http://php.net/manual/en/mysqli.quickstart.prepared-statements.php
You should use prepared statement to inject values into queries).
If you get any row then the email is not unique so redirect to the original form page, possibly displaying an error message.
If you don't get any record then you may proceed with the insert and then render a "mail registration succeeded" page.
Btw: you can't echo something and then set a header. As you start sending output header can't be set/changed.