[IMP] point_of_sale: better merge detection
[IMP] point_of_sale: better merge detection

If sells
1. product A
2. product B
3. product A

the lines of product A were not merged

Closes #25540

closes odoo/odoo#26602
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo