Use via layers which are determined in only one place, this is a cheap trick...

    /// we don't want ownership here permanently, so we don't use boost::ptr_vector

    std::vector<NET*>   nets;

    /// specctra cu layers, 0 based index:

    int     m_top_via_layer;

    int     m_bot_via_layer;

    /**

     * Function buildLayerMaps

    try

    {

        GetBoard()->SynchronizeNetsAndNetClasses();

        db.FromBOARD( GetBoard() );

        db.ExportPCB(  fullFileName, true );

                polygon->AppendPoint( lowerRight );

            }

            D( printf( "m_DeltaSize: %d,%d\n", aPad->GetDelta().x, aPad->GetDelta().y ); )

            // this string _must_ be unique for a given physical shape

            snprintf( name, sizeof(name), "Trapz%sPad_%.6gx%.6g_%c%.6gx%c%.6g_um",

                     uniqifier.c_str(), IU2um( aPad->GetSize().x ), IU2um( aPad->GetSize().y ),

    {

        NETCLASSES& nclasses = aBoard->m_NetClasses;

        // Assume the netclass vias are all the same kind of thru, blind, or buried vias.

        // This is in lieu of either having each netclass via have its own layer pair in

        // the netclass dialog, or such control in the specctra export dialog.

        // if( aBoard->GetDesignSettings().m_CurrentViaType == VIA_THROUGH )

        {

            m_top_via_layer = 0;       // first specctra cu layer is number zero.

            m_bot_via_layer = aBoard->GetCopperLayerCount()-1;

        }

        /*

        else

        {

            // again, should be in the BOARD:

            topLayer = kicadLayer2pcb[ GetScreen()->m_Route_Layer_TOP ];

            botLayer = kicadLayer2pcb[ GetScreen()->m_Route_Layer_BOTTOM ];

        }

        */

        // Add the via from the Default netclass first.  The via container

        // in pcb->library preserves the sequence of addition.

        NETCLASS*   netclass = nclasses.GetDefault();

        PADSTACK*   via = makeVia( netclass->GetViaDiameter(), netclass->GetViaDrill(),

                                   FIRST_LAYER, aBoard->GetCopperLayerCount()-1 );

                                   m_top_via_layer, m_bot_via_layer );

        // we AppendVia() this first one, there is no way it can be a duplicate,

        // the pcb->library via container is empty at this point.  After this,

        pcb->library->AppendVia( via );

#if 0

        // Stock vias have drill diameter of zero, this is not sensible to freerouter

        // I've seen no way to make stock vias useable by freerouter.  Also the

        // zero based diameter was leading to duplicates in the LookupVia() function.

        // User should use netclass based vias when going to freerouter.

        // output the stock vias, but preserve uniqueness in the via container by

        // Output the stock vias, but preserve uniqueness in the via container by

        // using LookupVia().

        for( unsigned i = 0; i < aBoard->m_ViasDimensionsList.size(); ++i )

        {

            int viaDrill    = aBoard->m_ViasDimensionsList[i].m_Drill;

            via = makeVia( viaSize, viaDrill,

                           FIRST_LAYER, aBoard->GetCopperLayerCount()-1 );

                           m_top_via_layer, m_bot_via_layer );

            // maybe add 'via' to the library, but only if unique.

            PADSTACK* registered = pcb->library->LookupVia( via );

            netclass = nc->second;

            via = makeVia( netclass->GetViaDiameter(), netclass->GetViaDrill(),

                           FIRST_LAYER, aBoard->GetCopperLayerCount()-1 );

                           m_top_via_layer, m_bot_via_layer );

            // maybe add 'via' to the library, but only if unique.

            PADSTACK* registered = pcb->library->LookupVia( via );

    //-----<export the existing real BOARD instantiated vias>-----------------

    {

        // export all of them for now, later we'll decide what controls we need

        // on this.

        // Export all vias, once per unique size and drill diameter combo.

        static const KICAD_T scanVIAs[] = { PCB_VIA_T, EOT };

        items.Collect( aBoard, scanVIAs );

    // this should never become a performance issue.

    PADSTACK* via = makeVia( aNetClass->GetViaDiameter(), aNetClass->GetViaDrill(),

                             FIRST_LAYER, aBoard->GetCopperLayerCount()-1 );

                             m_top_via_layer, m_bot_via_layer );

    snprintf( text, sizeof(text), "(use_via %s)", via->GetPadstackId().c_str() );

    clazz->circuit.push_back( text );