[FIX] account: refunding an invoice with two payment terms

Step to reproduce the bug:

Create a payment term with two separated due type:
-Due type 1: percent , value 30, numbers of days: 0 days after the invoice date
-Due type 2: Balance; value 0, Number of days 0: Last day of following month
Create an invoice with the payment term
Refund the invoice with the option "cancel: create refund and reconcile"
or "Modify: create refund, reconcile and create a new draft invoice".
Error message: "You are trying to reconcile some entries that are already reconciled!"
But no payment had been received and the receivable accounts were still open.

opw:694386

addons/account/wizard/account_invoice_refund.py

@@ -75,7 +75,7 @@ class AccountInvoiceRefund(models.TransientModel):
                     for tmpline in refund.move_id.line_ids:
                         if tmpline.account_id.id == inv.account_id.id:
                             to_reconcile_lines += tmpline
-                            to_reconcile_lines.reconcile()
+                            to_reconcile_lines.filtered(lambda l: l.reconciled == False).reconcile()
                     if mode == 'modify':
                         invoice = inv.read(
                                     ['name', 'type', 'number', 'reference',