Problem with showing featured image

Use an is_single() condition to ensure the thumbnail isn’t added on archive pages. You should also be using get_the_post_thumbnail and prepending it to $content, rather than just echo’ing it out:

function put_thumbnail_in_posting( $content ) { 
    if ( is_single() && has_post_thumbnail() && get_post_type() == 'post' ) {
        $thumbnail = get_the_post_thumbnail( null, '',
            array(
                'style' => implode( ' ',
                    array(
                        'padding: 6px;',
                        'margin-bottom: 8px;',
                        'margin-right: 8px;',
                        'max-width: 100%;',
                        'border: 1px solid rgb(204, 204, 204);',
                        'background: none repeat scroll 0% 0% rgb(238, 238, 238);',
                    )
                )
            )
        );

        $content = $thumbnail . $content;
    }

    return $content;
}