WordPress widget new instance creates content duplicates

I don’t see anything wrong with the widget class. The problem you have is that you are displaying the widget area, that contains the widgets, twice in your sidebar.php:

<aside id="secondary" class="widget-area col-sm-12 col-md-12 col-lg-12" role="complementary">
    <div class="col-lg-6 col-md-12">
    <?php dynamic_sidebar( 'main-widget' ); // this will display all the widgets in your main-widget sidebar ?>
</div>
    <div class="col-lg-6 col-md-12">
    <?php dynamic_sidebar( 'main-widget' ); // this will display again all the widgets in your main-widget sidebar ?>
    </div>
</aside><!-- #secondary -->

The second widget in each sidebar display is probably hidden due to some CSS, but I bet it is there, twice, once on the left and one on the right.

So, do not call dynamic_sidebar( 'main-widget' ); twice.

From what I understand you want to wrap the widgets in certain divs so you can put them on two columns. For this you need to filter the before-widget and after-widget params of each widget via the dynamic_sidebar_params filter. Or, better yet you could use some CSS to do the trick.

Here is a starter PHP code that will add wrappers to each widget in your target widget area:

/**
 * @param array $params
 */
function sole_add_widget_columns_wrapper( $params ) {
    // Add the opening wrapper tag
    $params[0]['before_widget'] = PHP_EOL . '<div class="col-lg-6 col-md-12">' . PHP_EOL . $params[0]['before_widget'];

    // Add the closing wrapper tag
    $params[0]['after_widget'] = $params[0]['after_widget'] . PHP_EOL . '</div><!-- close wrapper -->' . PHP_EOL;
}

/**
 * @param string $index
 */
function sole_handle_main_widget_area_columns( $index ) {
    // We only want to deal with the main widget area
    if ( 'main-widget' !== $index ) {
        return;
    }

    // Filter each widget params and add the wrappers
    add_filter( 'dynamic_sidebar_params', 'sole_add_widget_columns_wrapper', 10, 1 );

    // Hook an action to remove the filter above after we are done with this widget area.
    // This way we don't affect other widget area that may come, on the same page, after this one.
    add_action( 'dynamic_sidebar_after', 'sole_remove_main_widget_area_columns_filter', 10 );
}
add_action( 'dynamic_sidebar_before', 'sole_handle_main_widget_area_columns', 10 );

function sole_remove_main_widget_area_columns_filter() {
    remove_filter( 'dynamic_sidebar_params', 'sole_add_widget_columns_wrapper', 10 );
}

This code will wrap each widget, it will not put even and odd widgets into two column wrappers.