Lock the Schedule objects to ensure tasks are created once

Tested this with PostgreSQL and MySQL

django_q/cluster.py

@@ -491,7 +491,8 @@ def scheduler(broker=None):
         broker = get_broker()
     db.close_old_connections()
     try:
-        for s in Schedule.objects.exclude(repeats=0).filter(next_run__lt=timezone.now()):
+        with db.transaction.atomic():
+            for s in Schedule.objects.select_for_update().exclude(repeats=0).filter(next_run__lt=timezone.now()):
                 args = ()
                 kwargs = {}
                 # get args, kwargs and hook
@@ -529,9 +530,7 @@ def scheduler(broker=None):
                             next_run = next_run.shift(years=+1)
                         if Conf.CATCH_UP or next_run > arrow.utcnow():
                             break
-                # arrow always returns a tz aware datetime, and we don't want
-                # this when we explicitly configured django with USE_TZ=False
-                s.next_run = next_run.datetime if settings.USE_TZ else next_run.datetime.replace(tzinfo=None)
+                    s.next_run = next_run.datetime
                     s.repeats += -1
                 # send it to the cluster
                 q_options['broker'] = broker