[FIX] account: avoid base being included twice in group

Create multiple tax and assign them to the same tax group. Now create an
invoice, in the line add all the taxes in the group. Save and click on
"Preview". In the tax recap the tax group will be reported applied on a
total equal to the number of used taxes multiplied by the total of the
product. This append because in the calculation the tax are included
multiple times and not just once per group.

opw-2158989

closes odoo/odoo#42523

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>