Duplicate records in mysql

You need to return the result of your query in the function "conecta_bd". If the INSERT was a success, you display the success message, otherwise, you display another message saying what the error was.

function conectar_bd($query)
 {
     $DB_SERVER = "serverdb.cfe.mx";
     $DB_USER = "jair";
     $DB_DATABASE = "dbIPs";
     $DB_PWD = "123";

     $conn=mysqli_connect($DB_SERVER, $DB_USER,$DB_PWD, $DB_DATABASE);
     /* check connection */
     if (mysqli_connect_errno())
     {
         printf("Connect failed: %s\n" , mysqli_connect_error());
         exit();
     }
     $result=mysqli_query($conn,$query);
     if ($result)
         return 'Success';
     } else {
         return 'Failure'
     }
}

You also have to change these lines:

 conectar_bd($insert) ;

 echo "<center><br>Dirección IP dada de alta corectamente<br></center>";

Because of this:

 $result = conectar_bd($insert) ;
 if ($result == 'Success') {
     echo "<center><br>Dirección IP dada de alta corectamente<br></center>";
 } else {
     echo "<center><br>Dirección IP dada ya existe<br></center>";
 }

It is not a good idea to give the database a load of project logic, that creates additional complexity in the maintenance of the code. It is advisable to make a prior consultation (SELECT) asking if the IP address exists and if not, then do the INSERT.

Answering your question, modify this part:

conectar_bd($insert) ;
echo "<center><br>Dirección IP dada de alta corectamente<br></center>";

By:

if(conectar_bd($insert)) {
    echo "<center><br>Dirección IP dada de alta corectamente<br></center>";
} else {
    echo "<center><br>La direccion IP no ha podido ser insertada<br></center>";
}

What you could do in my opinion before the insert is to check that previously there is no record in the database that you are going to insert example:

//datos del post
$nombre = $_POST["nombre"];
$email = $_POST["email"];
$fecha = date("Y-m-d"); 

//COMPROBACION
// Comprobamos si el usuario esta registrado

$nuevo_usuario=mysql_query("select nombre from $tabla where nombre='$nombre'");
if(mysql_num_rows($nuevo_usuario)>0)
{
echo "
<p class='avisos'>El nombre de usuario ya esta registrado</p>
<p class='avisos'><a href='javascript:history.go(-1)' class='clase1'>Volver atrás</a></p>
";
}
// ------------ Si no esta registrado el usuario continua el script
else
{
// ==============================================
// Comprobamos si el email esta registrado

$nuevo_email=mysql_query("select email from $tabla where email='$email'");
if(mysql_num_rows($nuevo_email)>0)
{
echo "
<p class='avisos'>La direccion de e-mail ya esta registrada</p>
<p class='avisos'><a href='javascript:history.go(-1)' class='clase1'>Volver atrás</a></p>
";
}
// ------------ Si no esta registrado el e-mail continua el script
else
{
$result = mysql_db_query("$base_datos","insert into $tabla (nombre,email,fecha) values ('$nombre','$email','$fecha')"); 

include('cierra_db.php');

// Confirmamos que el registro ha sido insertado con exito

echo "
<p class='avisos'>Registro insertado con exito</p>
<p class='avisos'><a href='javascript:history.go(-1)' class='clase1'>Volver atrás</a></p>
";
} 
// ==============================================
} 

It's a small clear example, there are many ways to do it and optimize the code always before an insert you should have a method or function that you are going to consult in your db if the data exists that you are going to insert in case of existing since you return a message from the existing record you could show the user a message

"Registro existen desea modificarlo ?"

If the record does not exist insert it and notify the user of the correct insert! luck !!