Stopping duplicate entries in PHP form - php

Question

struggling to implement a way to stop duplicate entries within this code I've tried a few ways using an if and else statement but I can't seem to get it working. Can anyone provide a solution?
<?php
require('db.php');
// If form submitted, insert values into the database.
if (isset($_REQUEST['username'])){
// removes backslashes
$username = stripslashes($_REQUEST['username']);
//escapes special characters in a string
$username = mysqli_real_escape_string($con,$username);
$email = stripslashes($_REQUEST['email']);
$email = mysqli_real_escape_string($con,$email);
$password = stripslashes($_REQUEST['password']);
$password = mysqli_real_escape_string($con,$password);
$signupdate = date("Y-m-d H:i:s");
$query = "INSERT into `users` (username, password, email, signupdate)
VALUES ('$username', '".md5($password)."', '$email', '$signupdate')";
$result = mysqli_query($con,$query);
if($result){
echo "<div class='form'>
<h3>You are registered successfully.</h3>
<br/>Click here to <a href='login.php'>Login</a></div>";
}
}else{
?>
<div class="form">
<form name="registration" action="" method="post">
<input type="text" name="username" placeholder="Username" required />
<input type="email" name="email" placeholder="Email" required />
<input type="password" name="password" placeholder="Password" required />
<input type="submit" name="submit" value="Register" />
</form>
</div>
<?php } ?>

Answer 1

Replace
echo "<div class='form'>
<h3>You are registered successfully.</h3>
<br/>Click here to <a href='login.php'>Login</a></div>";
with
header("location:thankyou.php");
and add the text to the new page.
Disable the submit button after it was clicked
change
<input type="submit" name="submit" value="Register" />
with
<input type="submit" name="submit" value="Register" onClick="this.disabled=true; this.value='Processing…'";>
3.Change the email field in your database to unique
ALTER TABLE users ADD UNIQUE (email);
Add else to $result check:
if($result){
header("location:thankyou.php");
}
else{
echo "Registration failed, Make sure to use an email address that was not used before";
}

Answer 2

<?php
If(isset($_POST[‘submit]))
{
$username = stripslashes($_REQUEST['username']);
$username = mysqli_real_escape_string($con,$username);
$email = stripslashes($_REQUEST ['email']);
$email = mysqli_real_escape_string($con,$email);
$password = stripslashes($_REQUEST ['password']);
$password = mysqli_real_escape_string($con,$password);
$password = md5($password)
$signupdate = date("Y-m-d H:i:s");
$Check_username_indb_query = "select * from users where username = ‘$username’ ";
$result_check = mysqli_query($con,$query);
If(mysqli_fetch_rows($result_check)==1)
{
echo "Registration failed, this username is already taken please chose another one";
}
Else
{
$query = "INSERT into `users` (username, password, email, signupdate)
VALUES ('$username', '$password', '$email', '$signupdate')";
$result = mysqli_query($con,$query);
if($result)
{
echo "<div class='form'>
<h3>You are registered successfully.</h3>
<br/>
Click here to <a href='login.php'>Login</a></div>";
}
?>