Multiple pizza events in future causes crash

One-sentence description

Multiple pizza events in future causes crash

Current behaviour

When having multiple pizza events in the future the website crashes.

Expected behaviour

No crash, duh.

Steps to reproduce

  1. Create two pizza events in le future.
  2. Go to /api/v1/pizzas/event/ 
MultipleObjectsReturned at /api/v1/pizzas/event/
get() returned more than one PizzaEvent -- it returned 2!
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