diff --git a/openerp/osv/fields.py b/openerp/osv/fields.py
index 65c6c10e42c..464b203b29d 100644
--- a/openerp/osv/fields.py
+++ b/openerp/osv/fields.py
@@ -761,6 +761,15 @@ class many2many(_column):
             return
         rel, id1, id2 = self._sql_names(model)
         obj = model.pool.get(self._obj)
+
+        def link(ids):
+            # beware of duplicates when inserting
+            query = """ INSERT INTO {rel} ({id1}, {id2})
+                        (SELECT %s, unnest(%s)) EXCEPT (SELECT {id1}, {id2} FROM {rel} WHERE {id1}=%s)
+                    """.format(rel=rel, id1=id1, id2=id2)
+            for sub_ids in cr.split_for_in_conditions(ids):
+                cr.execute(query, (id, list(sub_ids), id))
+
         for act in values:
             if not (isinstance(act, list) or isinstance(act, tuple)) or not act:
                 continue
@@ -774,10 +783,7 @@ class many2many(_column):
             elif act[0] == 3:
                 cr.execute('delete from '+rel+' where ' + id1 + '=%s and '+ id2 + '=%s', (id, act[1]))
             elif act[0] == 4:
-                # following queries are in the same transaction - so should be relatively safe
-                cr.execute('SELECT 1 FROM '+rel+' WHERE '+id1+' = %s and '+id2+' = %s', (id, act[1]))
-                if not cr.fetchone():
-                    cr.execute('insert into '+rel+' ('+id1+','+id2+') values (%s,%s)', (id, act[1]))
+                link([act[1]])
             elif act[0] == 5:
                 cr.execute('delete from '+rel+' where ' + id1 + ' = %s', (id,))
             elif act[0] == 6:
@@ -789,8 +795,7 @@ class many2many(_column):
                     d1 = ''
                 cr.execute('delete from '+rel+' where '+id1+'=%s AND '+id2+' IN (SELECT '+rel+'.'+id2+' FROM '+rel+', '+','.join(tables)+' WHERE '+rel+'.'+id1+'=%s AND '+rel+'.'+id2+' = '+obj._table+'.id '+ d1 +')', [id, id]+d2)
 
-                for act_nbr in act[2]:
-                    cr.execute('insert into '+rel+' ('+id1+','+id2+') values (%s, %s)', (id, act_nbr))
+                link(act[2])
 
     #
     # TODO: use a name_search