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