More than one notify is sent to anonymous users if they use the same email with different case

If an anonymous user posts more than one comment using the same email BUT different case, comment_notify will send him more than one notify message.

I'll try to explain with an example:

1. Anonymous 1 posts a comment using anonymous1@example.org as email
2. when other users post comments, Anonymous 1 is correctly notified once for each comment
3. Anonymous 1 posts another comment, now using Anonymous1@example.org as email (note the capital A)
4. when other users post comments, Anonymous 1 is being notified twice: once at anonymous1@example.org and once at Anonymous1@example.org

_comment_notify_mailalert() function already contains a test to avoid multiple send to the same recipient, but this test is case-sensitive, so it does not work with my test case.