We separated the product name and it's description on devices so it looks like this:

As you can see the product's name "Some Product" is duplicated in the name-column.

I copied the function's head from account/account_invoice.py

I'm trying with a custom module, which looks like this:

# -*- coding: utf-8 -*-
from openerp import models
class account_invoice_line(models.Model):
    _inherit='account.invoice.line'
    @api.multi
    def product_id_change(self, product, uom_id, qty=0, name='', type='out_invoice',
            partner_id=False, fposition_id=False, price_unit=False, currency_id=False,
            company_id=None):
        res = super(account_invoice_line, self).product_id_change(
                product, uom_id, qty=qty, name=name, type=type, partner_id=partner_id,
                fposition_id=fposition_id, price_unit=price_unit,
                currency_id=currency_id, context=context, company_id=company_id)
        if product_id:
            product_obj = self.pool.get('product.product')
            product = product_obj.browse(cr, uid, product_id, context=context)
            if product.description_sale:
                res['value']['name'] = product.description_sale
            elif product.description_purchase:
                res['value']['name'] = product.description_purchase
        return res

After installing and trying to create an invoice position, I get this error, which I am not able to fix:

TypeError: product_id_change() got multiple values for keyword argument 'qty'