On submitting, you would get an array as if created like this:
$_POST['topdiameter'] = array( 'first value', 'second value' );
$_POST['bottomdiameter'] = array( 'first value', 'second value' );
However, I would suggest changing your form names to this format instead:
name="diameters[0][top]"
name="diameters[0][bottom]"
name="diameters[1][top]"
name="diameters[1][bottom]"
...
Using that format, it's much easier to loop through the values.
if ( isset( $_POST['diameters'] ) )
{
echo '<table>';
foreach ( $_POST['diameters'] as $diam )
{
// here you have access to $diam['top'] and $diam['bottom']
echo '<tr>';
echo ' <td>', $diam['top'], '</td>';
echo ' <td>', $diam['bottom'], '</td>';
echo '</tr>';
}
echo '</table>';
}