LNAFIN on "WP 4.5 and Twentyeleven Theme Problem"

PLEASE HELP ME!
I tried to solve the problem using Jonnexen's code.
Hopefully I did the right thing:
Downloaded the functions.php from twenty eleven theme, added at the end of the functions.php the snippet:
function wp36510_remove_singular_class( $classes ) {
$index = array_search( 'singular', $classes );
if ( false !== $index ) {
unset( $classes[ $index ] );
}

retu $classes;
}
add_filter( 'body_class', 'wp36510_remove_singular_class' );

and uploaded the functions.php into my child theme folder.

Then I tried to reload my website and got this error:

Fatal error: Cannot redeclare twentyeleven_excerpt_length() (previously declared in /homepages/46/d328509366/htdocs/fos_soziales/wp-content/themes/fos_soziales/functions.php:364) in /homepages/46/d328509366/htdocs/fos_soziales/wp-content/themes/twentyeleven/functions.php on line 366

Could please anybody help me?