Are you saying that your dropdown has duplicate authors?
If so, to remove duplicates, try:This is just a basic php array question - not a WP question.
function unique_authors ( $authors ) {
$newArray = array();
foreach( $authors as $item ) {
$itemArray = explode( ", ", $item );
$newArray = array_merge($newArray, $itemArray);
}
$newArray = array_unique($newArray);
return $newArray;
}
$authors = unique_authors( $authors );
foreach( $authors as $author ): //etc