I am having the same problem. I now have 13 Halloweens. If I uncheck Birthdays, then they dissappear, however, if I uncheck any one of my calendars, they also dissappear. It seems that if I have more then 4 calendars checked, then the multiple Holidays continue. I also want to have the Birthay reminder.
I have tinkered around with this and can reproduce the problem. I believe it is a bug in iOS with regard to syncing the "Contacts' birthdays and events" calendar through the Gmail account on your device. When you sync this calendar a new event is added to your device's calendar, but it somehow ends up being associated with the default calendar that you selected in your settings. With each successive sync of that calendar a new event is added and this is why the multiples appear. The number of multiple entries is the same as how many times you've synced the calendar. The same thing can be reproduced with the US Holidays calendar as well, so it seems to be related only to Shared Calendars in Google calendar.
I haven't found a workaround or fix yet. Right now the only option seems to be to turn off the "Contacts' birthdays and events" at https://www.google.com/calendar/iphoneselect
Use the inbuilt "Other/Birthdays" calendar instead. I sync contact information between iPhone, Gmail and Facebook using apps like Contacts Sync, so all my Gmail and iPhone contacts are identical and the birthdays are in my iPhone contacts. You can still use iCloud to backup and access your contacts if you use the syncing app.
Now to remove those multiple entries. Turn off (uncheck) the "Contacts' birthdays and events" at https://www.google.com/calendar/iphoneselect. Now on your device go into Settings/Mail, Contacts Calendars, select your Gmail account, turn off the calendars and select "Delete from my iPhone". Now open the calendar app (the multiples are still there), select "Calendars" at top left, click the little blue arrow at the right of On My Phone/Calendar, and select Delete Calendar. Now go back into Settings and turn on the calendars on your Gmail account again. The duplicates should be gone. You can go and turn on the Other/Birthdays calendar in the calendar app now if you hadn't before. You may also need to set your default calendar again in Settings/Mail, Contacts, Calendars. Let me know how that goes.
Hopefully apple will come up with a fix for this bug, which seems to have been ongoing for a couple of years now judging from all the posts about it all over the web.
Sorry, this was said before, but apparently there is no way to delete posts:
While it's true that having less than 4 calendars doesn't show the multiplied birthdays any more, they are still there on the phone and take up memory. As multiplication is endless, they will eventually take all memory (several gigabytes) even though they are not shown on the Calendar.
iOS 6.0.1 iPhone 4
I'm having the same issue, only with repeating events as oppose to repeating birthdays. I've followed the steps in a similar discussion for solving it by going into settings and calendar, doing the select/deselect, delete/resynchonize all the calendars I'm using (google: personal, US holidays; facebook; and On My iPhone's Calendar) and it's either a bug with the On My iPhone's calendar, facebook's calendar or both.
I believe this because I tried going into Settings/Mail, Contacts, Calendars, turning off calendars, delete from my phone, etc. and it eventually leads back to duplicating events. I then tried leaving my gmail calendars deleted from my phone, went into Calendar, selected "calendars" on the top left, and tried checking/unchecking the remaining calendars (Facebook Events&Birthdays; Other Birthdays; On My iPhone Calendar), and yet the duplicated events reappear. It's whenever I go into the Calendar app, choose "Calendars" on the top left, and On My iPhone/Calendar is checked along with Facebook Events AND Facebook Birthdays (with or without Other Birthdays being checked).
The US Holidays events keep duplicating no matter how many times I delete the calendar using the little blue arrow on Calendar, under On My iPhone, in "Calendars" in the Calendar app. The events just keep reappearing, duplicated, whenever I check On My iPhone Calendar, Facebook Events and Facebook Birthdays, all three together. Please fix this. Thanks!
I am not 100% sure I understood what you have actually tried. But in my experience, the only failsafe way to delete duplicate calendar entries is to reset the phone. Because these calendar entries somehow get disassociated with the calendar they originate from, deleting this calendar won't cure the issue. Deleting all calendars in the way suggested by Dave TBar might help, but this assumes that all duplicate events get associated with the On My iPhone calendar, which might be true or might not.
So I'd suggest you delete all calendar accoutns, make a full backup of your phone, reset it to factory defaults and restore it from the backup. This is what I will do tonight probably. I haven't tried yet, so I can only hope that the backup won't contain all the stray events.
My best guess from reading your post is that the US Holidays calendar keeps reappearing because you haven't unchecked it at https://www.google.com/calendar/iphoneselect. Go to that site and uncheck it. Make sure you hit the save button at bottom right. Then sync your calendar app. Force closing and reopening the app helps if it won't sync.
Checking or unchecking calendars in the iPhone Calendar app only changes whether they are displayed on the calendar or not, it does not control whether they are synced or not. Also, deleting and resyncing your Google calendars will not remove the duplicates because they are no longer associated with your Google calendars.
Click on one of your duplicated events and see what calendar it is associated with. Is it the calendar it should be on, or the calendar you selecteed as default in Setting/Mail, Contacts, Calendars? This may help in figuring out what is wrong. Many people have said they see these duplicates only when multiple calendars are selected, but this is not what I've observed. Also, from what I can tell, all events are always associated with a calendar in the phone. Even if you delete all synced calendars the phone will still have a default calendar and events will be moved into that calendar, so deleting all events from it fixes the problem.
Like I've said, it seems the only work around right now is not to sync any Google calendars which lead to duplicates such as Contacts Birthdays and US Holidays.
Let me know how you go
Thank you Dave. Deselecting US Holidays on google.com/calendar/iphoneselect is eventually the method I used to prevent duplicate entries from appearing on my Iphone's calendar. It's just strange that the events were duplicating when I left google calendar un-synchronized all together. And the US Holiday events do appear now without duplicate entries, only now, they're associated with my personal google Calendar - which is reflected by the similar colored dot as the personal entries into my google Calendar (whether it be entered through my phone or on google.com/calendar).
Checking which calendar the duplicated events were associated with, I found that they were in the phone's memory - On My iPhone Calendar - because it said it was associated with it AND it showed the same color dot no matter to which color I changed On My iPhone Calendar. When I tried hitting the "little blue arrow" and deleted On My iPhone's Calendar, the delete button didn't seem to work (it deleted it, but somehow the phone brings up the duplicate events once I re-checked On My iPhone Calendar, Facebook Events and Facebook Birthdays, all three together).
Now, if I go into any day with a US Holiday event - it first appears to have no colored dot. Selecting the event to see its details, then going back to the day of the calendar using the arrow at the top left, brings the color dot showing the US Holiday event has been associated with my personal google Calendar.
Now get this: now that I've left US Holidays on google.com/calendar/iphoneselect unselected, like I said, US Holidays appears as my personal Google Calendar, when I go into any US Holiday event it first appears to have no colored dot. Now, when I select the event to see its details, then select "Calendar (my personal name) >" to see the calendars it can possibly associate with, then immediately going back to just the event details... the app crashes and closes. No particular reason why I'm doing the last shuffling between menus, other than wanting to see the event with a colored dot (to see its association), but it sure seems like a bug.
For any matter, the duplicate entries has been resolved, so thank you.
I found a solution to get rid of all of the data that it has taken up the "Other" space on my iPhone (which was caused by the calendar sync's). The link is http://www.thinkcomputers.org/how-to-remove-other-data-from-your-iphone/ I followed "The DiskAid Solution", and it worked! Hope this works for everyone else!
I tried some of these workarounds and settings that seemed to make the duplicate events disappear, but continued to have lots of problems with the calendar being sluggish, battery draining, backups taking a long time, changing calendar settings causing crashes/hangs, etc. The cause of my problems was that the calendar database was huge and, presumably, badly corrupted. For those of you that think you have corrected the problems, I would suggest you do the following to be sure:
backup your device via iTunes
look in the backup folder - ~/Library/Application Support/MobileSync/Backup/<device UUID> in OS X
the calendar database file, SHA-1 hash of HomeDomain-Library/Calendar/Calendar.sqlitedb, is
if it is abnormally large - mine was just under 1 GB (yes G) before fixing and is now 471 KB after correcting it and restoring all my calendars - you may want to try to really fix the problem.
The most straightforward way would be to use iTunes to do a sync, transfer purchases from device to computer, do a 'restore device', and then setup the device as 'new' (there may be a way to do this with iCloud too). This is not so good if you have a lot of custom settings and data in your apps though. I finally figured out how to replace the calendar database in the backup with a good (but empty) version and restore my device from that backup. Then I only had to reenter the calendar info. That process (at least the way I did it) was somewhat tricky (involved doing unencryped backups, restoring device (to get good calendar db), editing the Manifest.mbdb file with new calendar db info using a hex editor, ...) and I don't have the time right now to try to write it down carefully. I may try later if there seems to be interest.
I just 'fixed' mine today and it does seem better so far. For now, I have also turned off the syncing of 'Contacts birthdays and events' calendar at the 'iphoneselect' website (not really needed since all my calendar apps already have a 'birthday' calendar I can use), turned off 'Shared Calendar Alerts' in the iOS calendar settings (I wonder if this my have been the root cause of the corrupted database - though I have seen reports that turning this off does not actually do anything), and am only using a GoogleSync calendar account (before I had both GoogleSync and calDAV accounts set up). I am hoping it lasts until Apple and Google get their acts together and fix their respective problems (I suspect both have serious bugs in their calendar syncing codes).
Just wanted to confirm that removing these duplicate stray calendar entries is not easy, and in my experience only restoring the iDevice to factory settings works.
@Dave TBar: In my case at least, removing the local calendar from the calendar app doesn't work. I can select it, delete it, confirm deletion, and then it takes a while to apparently complete the deletion. However, the deleted local calendar is still there, regardless of how often I delete it.
Leaving it where it is and then setting up a new GMail synced calendar should also do the trick because iOS asks if the synced calendar should replace all local calendar data. Even chosing Yes does not, however, delete the stray data.
Deleting the iDevice and restoring it from a backup also copies all stray calendar entries back to the device, even when, before backing up the device, I deleted all calendars from the device. This is where would_rtfm_if_it_existed's suggestion just above would probably do the trick, but that involves too much messing around with the data base at least for me.
So I will once more restore my iPad mini to factory settings tonight and re-enter all accounts and other personal data manually.
What I would really like to know is what is the cause of the trouble. Over at the Google forums, a developer admitted they were messing around with calendar sync in a way that would have probably caused the issue. I really cannot imagine that's true, especially because under no circumstances should corrupted caledar data offered to for sync by Google cause a corrupt calendar data base on the device. iOS should just discard corrupt data. And in addition I can't believe they are messing around with the calendar in such a way.
Because none of the above issues can be experienced with any iOS version earlier than iOS 6, I am pretty sure that Apple hosed calendar sync. Unfortunately, at least the German Applecare is unable to reproduce the issue because they cannot use anything but Apple products and services, so they can't even check if there is any syncing issue with GMail (or any other service other than iCloud). This is not really helpful.
UPDATE: So far my device (iPad 2) is working fine after my fix - except for one thing. I noticed that all the events in my secondary calendars were appearing as uneditable invitations (apparently something that started with iOS 6.0.1 if other discussions are to be trusted). As a result I started looking at Google's current syncing recommendations. For calendars, they now only specify CalDAV for syncing with Apple devices (and the new name for the site for selecting the synced calendars is now 'syncselect' rather than 'iphoneselect' (though both names still work)). Looking further, I saw that they have issued an end of life for GoogleSync support - http://support.google.com/a/bin/answer.py?hl=en&answer=2716936#gcs
So I turned off calendars in my GoogleSync account and activated my CalDAV account (I did this as a separate account, but I think you can do it in conjunction with a gmail account too). I think I may lose a few features with calDAV vs GoogleSync, but 'working' is the most important feature. Next I guess I will have to see about moving my email out of GoogleSync too - aargh.