code: public static void avoidDoubleBorders (@Nonnull final PLTable ret)
    {
      boolean bPreviousRowHasBottomBorder = false;
      for (int i = 0; i < ret.getRowCount (); i++)
      {
        boolean bRowHasBottomBorder = true;
        boolean bRowHasTopBorder = true;
        final PLTableRow aRow = ret.getRowAtIndex (i);
        for (int j = 0; j < aRow.getColumnCount (); j++)
        {
          final PLTableCell aCell = aRow.getCellAtIndex (j);
  
          if (aCell.getBorderBottomWidth () == 0)
            bRowHasBottomBorder = false;
          if (aCell.getBorderTopWidth () == 0)
            bRowHasTopBorder = false;
        }
        if (bPreviousRowHasBottomBorder && bRowHasTopBorder)
          aRow.setBorderTop (null);
        bPreviousRowHasBottomBorder = bRowHasBottomBorder;
      }
    }
docs: If two joined rows both have borders at their connecting side, the doubles
  width has to be removed
  
  @param ret
  the PLTable, whose doubled borders are to be removed