[FIX] account: prevent invoice line duplication on partner duplication

If a company contact (a partner with a company set as parent) had invoices, and the company of this contact was duplicated, all the invoices lines were duplicated,  on the original invoice moreover (new lines were added on existing invoices)
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32