diff --git a/invoice.py b/invoice.py
index 1339a6882f..9608d42376 100644
--- a/invoice.py
+++ b/invoice.py
@@ -18,7 +18,8 @@ def process_sale(func):
         pool = Pool()
         Sale = pool.get('sale.sale')
         with Transaction().set_context(_check_access=False):
-            sales = [s for i in cls.browse(invoices) for s in i.sales]
+            sales = Sale.browse(
+                set(s for i in cls.browse(invoices) for s in i.sales))
         func(cls, invoices)
         with Transaction().set_context(_check_access=False):
             Sale.process(sales)
@@ -158,7 +159,7 @@ class InvoiceLine:
             lines = cls.browse(lines)
             invoices = (l.invoice for l in lines
                 if l.type == 'line' and l.invoice)
-            sales = [s for i in invoices for s in i.sales]
+            sales = Sale.browse(set(s for i in invoices for s in i.sales))
         super(InvoiceLine, cls).delete(lines)
         with Transaction().set_context(_check_access=False):
             Sale.process(sales)