Exchange calendar listed twice
Following the discussion suggested in the comments I was able to find a partial solution:


In the Internet Accounts preferences (system-wide) uncheck the Calendars option from the Exchange account
In the preferences of the Calendars application choose the Accounts tab and disable the only visible account
In the system-wide Internet Accounts preferences re-enable the exchange account


The only drawback is that you will still see a disable Exchange account in your Calendars preferences.