Block registration if email already exists in mysqli database - php

Question

Users can use the same email address for different registration.
Tried using different solutions from developers on Stack Overflow who have experienced the same problems that I have, including making the emailUsers column a UNIQUE KEY in the database table, and implementing some sort of jQuery, but not successful. Think the problem is I am missing an 'else if' block for duplicate email, but all the various ways I tried to implement this, it does not do anything.
<div id="signup">
<div class="container">
<div class="row">
<div class="col-sm-6 offset-sm-1">
<div class="signup-left position-fixed text-center">
<img src="img/200.png">
<br><br><br>
<?php
if(isset($_GET['error']))
{
if($_GET['error'] == 'emptyfields')
{
echo '<div class="alert alert-danger" role="alert">
<strong>Error: </strong> Fill In All The Fields
</div>';
}
else if ($_GET['error'] == 'invalidmailuid')
{
echo '<div class="alert alert-danger" role="alert">
<strong>Error: </strong> Please enter a valid email and user name
</div>';
}
else if ($_GET['error'] == 'invalidmail')
{
echo '<div class="alert alert-danger" role="alert">
<strong>Error: </strong> Please enter a valid email
</div>';
}
else if ($_GET['error'] == 'invaliduid')
{
echo '<div class="alert alert-danger" role="alert">
<strong>Error: </strong> Please enter a valid user name
</div>';
}
else if ($_GET['error'] == 'passwordcheck')
{
echo '<div class="alert alert-danger" role="alert">
<strong>Error: </strong> Passwords donot match
</div>';
}
else if ($_GET['error'] == 'usertaken')
{
echo '<div class="alert alert-danger" role="alert">
<strong>Error: </strong> This User name is already taken
</div>';
}
else if ($_GET['error'] == 'invalidimagetype')
{
echo '<div class="alert alert-danger" role="alert">
<strong>Error: </strong> Invalid image type
</div>';
}
else if ($_GET['error'] == 'imguploaderror')
{
echo '<div class="alert alert-danger" role="alert">
<strong>Error: </strong> Image upload error, please try again
</div>';
}
else if ($_GET['error'] == 'imgsizeexceeded')
{
echo '<div class="alert alert-danger" role="alert">
<strong>Error: </strong> Image too large
</div>';
}
else if ($_GET['error'] == 'sqlerror')
{
echo '<div class="alert alert-danger" role="alert">
<strong>Website Error: </strong> Contact admin to have the issue fixed
</div>';
}
}
else if (isset($_GET['signup']) == 'success')
{
echo '<div class="alert alert-success" role="alert">
<strong>Signup Successful</strong> Please Login from the login menu
</div>';
}
?>
<form id="signup-form" action="includes/signup.inc.php" method='post' enctype="multipart/form-data">
<input type="submit" class="btn btn-light btn-lg" name="signup-submit" value="Signup">
<br><br>
<a href="login.php">
<i class="fa fa-sign-in fa-2x social-icon" aria-hidden="true"></i>
</a>
<a href="contact.php">
<i class="fa fa-envelope fa-2x social-icon" aria-hidden="true"></i>
</a>
</div>
</div>
I want to be able to block a user from registering with the same email, and to generate an error message when the user tries to do this.
EXTRA INFO: Can post further, detailed methods of me trying to make the emailUsers column a UNIQUE KEY, but when I do this, new users can no longer register, even if they use a different email.