I have the same problem, but in my case the solution was to log into icloud and drag the calender to the BOTTOM not the TOP as suggested in other posts. (inconvenient of course)
Its a bug obviously, since OSX-preferences has a place to define the default calender, but equally obviously noone from Apple monitors these forums or anywhere else you could report a bug :-(
I only have one Exchange and two Google Calendars setup.
Renamed the Exchange to appear first in Settings.
Defined the Exchange Calendar as Default.
None of these calendars appear online in iCloud, so I couldn't change any preferences there.
When I click on an ICS file in my Exchange mail account, it opens the invite in Calendar using my Google Account. When I right-click the event, I can only move it to the second Google account. When I drag the event to my Exchange calendar, the event is duplicated in Exchange (without the Accept/Reject options). From there, if I try to delete the original Google invite, I'm asked if I'm sure I want to decline the event.
Didn't solve for me using iCloud. After a long time of searching plists, settings, etc. I finally found that when I reordered my calendars on iCloud, then the meeting invite would be created in different calendars. Strangely I found that the "default" meeting invite calendar (when receiving an invite attachment via email) is stuck to go to the 3rd position on my calendar list in iCloud. Whichever calendar was third down on the list, would get the meeting. I played around for a long time reordering them, hoping it would clear out some bad setting, and hoping it would adhere to either the default calendar setting or the alphabetical rule or the first in the list rule...but in the end, I just put the calendar I wanted as the default meeting invite recipient in the 3rd slot.
I have same issue - Exchange work calendar invites are automatically accepted from mail.
My exchange work calendar is NOT iCal, so there is no way to move it to the top of the list.
The ONLY workaround I've found is to accept them from iOS device (they work perfectly, and update appropriately).
I changed the setting in mail to "Add invitations to Calendar", set to 'never'.
As long as Calendar has the exchange account set to refresh, they will show up (changes too).
Crappy workaround - CMON Apple. Fix this, please!!!!!!
I'm having the same issue - when I accept an Exchange invitation in Mail, a duplicate event is created in my Gmail account in Calendar.
I've filed a bug report with Apple (#: 17434464). If any of you have the same issue, please file a bug report at bugreport.apple.com - contrary to popular belief, every single one of these reports is read. The more they get, they more likely an issue will be fixed. Forums are not the best way to report bugs, other than to verify that others have the same issue. I reported the issue under 'Mail' as opposed to 'Calendar' because it appears that's where the issue is.