**Impacted versions:**
* 9.0

**Steps to reproduce:**
* Enable multi currencies
* Make sure that the current rate of a foreign currency (i.e. EUR) is not 1.0 (i.e. 0.84)
* Create a purchasable product with a cost (i.e. 33$)
* Create a vendor bill in a foreign currency (i.e. EUR)
* Add the product on the invoice

**Current behavior:**
* The unit price will be 23.28 == (33 * 0.84) * 0.84

**Expected behavior:**
* The unit price should be 27,72 == 33 * 0.84
* Probably due to this line of code: https://github.com/odoo/odoo/blob/9.0/addons/account/models/account_invoice.py#L1241

**Video/Screenshot link (optional):**
* On Runbot Community 9.0: https://www.youtube.com/watch?v=60qPmczo5vk


for invoice in self:
            
            
            if invoice.type in ('in_invoice', 'in_refund') and invoice.reference:
                if self.search([('type', '=', invoice.type), ('reference', '=', invoice.reference), ('company_id', '=', invoice.company_id.id), ('commercial_partner_id', '=', invoice.commercial_partner_id.id), ('id', '!=', invoice.id)]):
                    raise UserError(_("Duplicated vendor reference detected. You probably encoded twice the same vendor bill/refund."))
        return self.write({'state': 'open'})
    @api.model
    def line_get_convert(self, line, part):