## Bug description
- See screenshot below (it's likely being processed in 3 workers at the same time).

![sc](https://user-images.githubusercontent.com/5864173/92016443-f6457800-ed17-11ea-9b7f-959a3306850e.png)


## Expected behavior
- We should receive the event only once.


## How to reproduce

1. View a user of recent creation with this event ([example](https://app.posthog.com/person/5fabb7b9-a36c-4419-af97-eea75960b2e4)) 

## Environment

- Cloud from PR #1508 

#### *Thank you* for your bug report – we love squashing them!

posthog/celery.py
if not settings.DEBUG:
        sender.add_periodic_task(1.0, redis_celery_queue_depth.s(), name="1 sec queue probe", priority=0)
    
    sender.add_periodic_task(10.0, redis_heartbeat.s(), name="10 sec heartbeat", priority=0)
    
    sender.add_periodic_task(
        crontab(day_of_week="mon,fri"), update_event_partitions.s(),  
    )
    if getattr(settings, "MULTI_TENANCY", False) or os.environ.get("SESSION_RECORDING_RETENTION_CRONJOB", False):
        sender.add_periodic_task(crontab(minute=0, hour="*/12"), run_session_recording_retention.s())
    
    if not getattr(settings, "MULTI_TENANCY", False):
        sender.add_periodic_task(crontab(day_of_week="mon"), status_report.s())