Holiday App allows to select the same interval multiple times

Steps to reproduce:

1. Create a new Holiday request with an interval (3 days)
2. Create another request with the same exact dates as the first request
3. Approve both requests
4. Check the Overview (HolidayRequest.Summary) - it doubles the number of days. "Days taken" would display "6 days".

It shouldn't be possible to chose the same exact interval twice.
If the interval includes a date already taken, it should be unavailable and excluded from the number of days of the new request.