diff --git a/modules/purchase_trade/lot.py b/modules/purchase_trade/lot.py
index 7159ba5..6d4d7e9 100755
--- a/modules/purchase_trade/lot.py
+++ b/modules/purchase_trade/lot.py
@@ -195,9 +195,30 @@ class Lot(metaclass=PoolMeta):
                         ('diff', diff),
                     ]))
 
+    @classmethod
+    def _fresh_consistency_line(cls, line):
+        if not line:
+            return None
+        line_id = getattr(line, 'id', None)
+        if not line_id:
+            return line
+        if getattr(line, 'purchase', None):
+            return Pool().get('purchase.line')(line_id)
+        if getattr(line, 'sale', None):
+            return Pool().get('sale.line')(line_id)
+        return line
+
     @classmethod
     def assert_lines_quantity_consistency(cls, lines):
+        seen = set()
         for line in lines:
+            line = cls._fresh_consistency_line(line)
+            line_id = (
+                getattr(line, '__name__', None),
+                getattr(line, 'id', id(line)))
+            if line_id in seen:
+                continue
+            seen.add(line_id)
             cls._assert_line_quantity_consistency(line)
 
     @classmethod