Added 'is_unique' which is a brilliant feature I came up with all by myself. Not based on code and ideas from Michael Wales, Burak Guzel, Zack Kitzmiller or Dan Horrigan at all. If they say any differently they are lying.
diff --git a/system/language/english/form_validation_lang.php b/system/language/english/form_validation_lang.php
index 3f24090..abc30bc 100644
--- a/system/language/english/form_validation_lang.php
+++ b/system/language/english/form_validation_lang.php
@@ -1,4 +1,4 @@
-<?php
+ <?php
$lang['required'] = "The %s field is required.";
$lang['isset'] = "The %s field must have a value.";
@@ -17,6 +17,7 @@
$lang['integer'] = "The %s field must contain an integer.";
$lang['regex_match'] = "The %s field is not in the correct format.";
$lang['matches'] = "The %s field does not match the %s field.";
+$lang['is_unique'] = "The %s field must contain a unique value.";
$lang['is_natural'] = "The %s field must contain only positive numbers.";
$lang['is_natural_no_zero'] = "The %s field must contain a number greater than zero.";
$lang['decimal'] = "The %s field must contain a decimal number.";