Create.php

<?php
session_start();
include_once 'conexao.php';

$nome = filter_input(INPUT_POST, 'nome', FILTER_SANITIZE_SPECIAL_CHARS);
$email = filter_input(INPUT_POST, 'email', FILTER_VALIDATE_EMAIL);
$logradouro = filter_input(INPUT_POST, 'logradouro', FILTER_SANITIZE_SPECIAL_CHARS);
$cep = filter_input(INPUT_POST, 'cep', FILTER_SANITIZE_NUMBER_INT);
$numero = filter_input(INPUT_POST, 'numero', FILTER_SANITIZE_NUMBER_INT);
$complemento = filter_input(INPUT_POST, 'complemento', FILTER_SANITIZE_SPECIAL_CHARS);
$bairro = filter_input(INPUT_POST, 'bairro', FILTER_SANITIZE_NUMBER_INT);
$celular = filter_input(INPUT_POST, 'celular', FILTER_SANITIZE_NUMBER_INT);
$telefone = filter_input(INPUT_POST, 'telefone', FILTER_SANITIZE_NUMBER_INT);

$querySelect = $link->query("select email from Clientes");
$array_emails = [];

while ($emails = $querySelect->fetch_assoc()):
    $emails_existentes = $emails['email'];
    array_push($array_emails, $emails_existentes);


endwhile;

if(in_array($email,$array_emails)):
    $_SESSION['msg'] = "<p class='center red-text'>".'Já existe um cliente cadastrado com esse email'."</p";
    header("Location../");
else:
    $queryInsert = $link->query("insert into Clientes values(default,'$nome','$email','$logradouro','$cep','$numero','$complemento','$bairro','$celular','$telefone'");
    $affected_rows = mysqli_affected_rows($link);

    if($affected_rows > 0):
        $_SESSION['msg'] = "<p> class='center green-text'>".'Cadastro efetuado com sucesso'."<br>";
        header("Location:../");
    endif;
endif;