<?php
$nome=$_POST['nome'];
$query=mysqli_query($connection, "SELECT * FROM tabella WHERE campo1='$nome'");
$num=mysqli_num_rows($query);
if($num==0){
$query1=mysqli_query($connection, "INSERT INTO tabella VALUES ('$nome')");
} else{
echo "Dato già esistente";
}
?>
<input type="text" name="nome">