check for duplicate entry vs use PDO errorInfo result

I have a MySQL table which has a field for email addresses which is defined as unique. For this example, let's say that all my form does is allow a user to insert their email address into the table.

Since the email field is unique, the query should fail should they try to enter the same email twice. I'm curious about the trade-offs between between the two scenarios:

1) Run a quick SELECT statement before performing the insert. If the select returns results, inform the user, and do not run the INSERT statement.

2) Run the INSERT statement, and check for a duplicate entry error

// snippet uses PDO
if (!$prep->execute($values))
{
    $err = $prep->errorInfo();
    if (isset($err[1]))
    {
        // 1062 - Duplicate entry
        if ($err[1] == 1062)
            echo 'This email already exists.';
    }
}

Also, please assume normal use, meaning that duplicate entries should be minimal. Therefore, in the first scenario you obviously have the overhead of running an additional query for every insert, whereas in the second you're relying on error handling.

Also, I'm curious to hear thoughts on coding style. My heart says 'Be a defensive programmer! Check the data before you insert!' while my brain says 'Hmmm, maybe it's better to let MySQL take care of checking the data for you'.

EDIT - Please note this isn't a "How do I do this" question, but rather a "Why should I do this a particular way" question. The little code snippet I included works, but what I'm curious about is the best way to solve the problem.

INSERT + check status should be a better approach. With SELECT + INSERT you can have another thread insert the same value between the SELECT and the INSERT, which means you would need to also wrap those two statements in a table lock.

It is easy to err on the side of too much defense in your coding. Python has the saying that "it is easier to ask for forgiveness than to ask for permission", and this philosophy is not really Python-specific.