## 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
from posthog.tasks.session_recording_retention import session_recording_retention_scheduler
    session_recording_retention_scheduler()
@app.task(ignore_result=True)
def calculate_event_action_mappings():
    from posthog.tasks.calculate_action import calculate_actions_from_last_calculation
    calculate_actions_from_last_calculation()
@app.task(ignore_result=True)
def calculate_cohort():
    from posthog.tasks.calculate_cohort import calculate_cohorts
    calculate_cohorts()
@app.task(ignore_result=True)
def check_cached_items():
    from posthog.tasks.update_cache import update_cached_items
    update_cached_items()
@app.task(ignore_result=True)
def update_cache_item_task(key: str, cache_type: str, payload: dict) -> None:
    from posthog.tasks.update_cache import update_cache_item
    update_cache_item(key, cache_type, payload)
@app.task(ignore_result=True)
def send_weekly_email_report():
    if settings.EMAIL_REPORTS_ENABLED:
        from posthog.tasks.email import send_weekly_email_reports
        send_weekly_email_reports()
@app.task(ignore_result=True, bind=True)
def debug_task(self):
    print("Request: {0!r}".format(self.request))