Quoting the forum thread:

"2) When displaying two boxes handling crossing the IDL there may have duplicate objects from left and right box - this duplicates must be erased, otherwise they will be drawn twice. This effect can be seen on the semitransparent objects, such as user-opened files (kml for example) with alpha channel in fill polygon colors. Issue occurs in function GeoGraphicsScene::items (file GeoGraphicsScene.cpp)."

Proposed fix:
QList< GeoGraphicsItem* > GeoGraphicsScene::items( const Marble::GeoDataLatLonAltBox& box, int zoomLevel ) const
{
    if ( box.west() > box.east() ) {
        // Handle boxes crossing the IDL by splitting it into two separate boxes
        GeoDataLatLonAltBox left;
        left.setWest( -M_PI );
        left.setEast( box.east() );
        left.setNorth( box.north() );
        left.setSouth( box.south() );

        GeoDataLatLonAltBox right;
        right.setWest( box.west() );
        right.setEast( M_PI );
        right.setNorth( box.north() );
        right.setSouth( box.south() );

///-        return items( left, zoomLevel ) + items( right, zoomLevel );
        QList<GeoGraphicsItem *> leftItems = items( left, zoomLevel );      ///+
        QList<GeoGraphicsItem *> rightItems = items( right, zoomLevel );    ///+
        for( int i = 0; i < leftItems.size(); i++ ) {                       ///+
            for( int j = 0; j < rightItems.size(); ) {                      ///+
                if( leftItems.at( i ) == rightItems.at( j ) ) {             ///+
                    rightItems.removeAt( j );                               ///+
                    continue;                                               ///+
                }                                                           ///+
                j++;                                                        ///+
            }                                                               ///+
        }                                                                   ///+
        return leftItems + rightItems;                                      ///+
    }

    <...>

    return result;
}