previous/next attachment image issue

I know the function to call up the navigation

<?php next_image_link( $size, $text ); ?>
<?php previous_image_link( $size, $text ); ?>

…but it does not work anymore 🙁

I’ll explain until a few days worked but now that I have added more images to the library the code does not work and locks the page
how I solved it?

…erasing the last images works 🙁

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Error_log

[12-Jan-2015 19:52:17] PHP Fatal error: Cannot redeclare next_image_link() (previously declared in /wp-includes/media.php:1912) in /themes/functions.php on line 1011