Double anniversaries

Turn off iCloud Contacts, Calendar, or Reminders and turn it back on

1. Tap Settings > [your name] > iCloud. (If you're using iOS 10.2 or earlier, go to Settings > iCloud.)
2. Slide to turn off Contacts, Calendars, and Reminders.
3. If you see your calendars or reminders on iCloud.com or any of your devices, you can choose Delete from My [device]. If not, choose Keep on My [device].
4. Wait a few minutes, then turn on Contacts, Calendar, or Reminders again.

Restart your iPhone, iPad, or iPod touch

Restart your device. If you're not sure how to do this, follow these steps.

Welcome to the Apple Support Communities!

I understand that you seem to be having a problem with duplicated anniversaries in your Calendar app. I would like to help you with some articles that might help you with this issue.
I want to offer you the following article with additional info:
Get help with duplicate iCloud calendars​
That article also offers you information to a related article that can help to determine which calendar might be repeated:
iCloud: Advanced Calendar and iCal troubleshooting
Pay attention to the following portion of that article:

Remove unusable and duplicate events

Duplicate events, especially those that occurred in the past, can slow down Calendar (or iCal) particularly when syncing with iCloud. You can use some simple tricks in iCal to find and remove duplicated data.

1. Open Calendar (or iCal).
2. Type a period in the Search field located in the top-right corner.
3. All of your calendar events will appear in the search results, located in the side of the Calendar window (or bottom of the iCal window). Depending on how many events you have, it may take a few minutes for all of them to display.
4. Scroll through the events. Remove any events that do not have titles by selecting them, and choosing Delete from the Edit menu.

   You can select consecutive items by holding down the Shift key while selecting them. You can select non-consecutive items by holding down the Command key while selecting them.

5. Remove any obvious duplicate events. You may want to retain a single version of a duplicate event, but you should remove unwanted versions.
6. Look for any events whose titles contain complicated characters or symbols. These include high ASCII or double-byte (Unicode) characters (such as Japanese, Chinese, Korean, Emoji, or other extended ASCII characters). Try removing these characters or substituting them with simpler characters. For example, if you had an event titled " Store Appointment" rename it to "Apple Store Appointment".
7. Click the Date field in the search results to sort by date.
8. Scroll through the events and scan for duplicates again. In many cases, it will be easier to locate duplicate events that exist on the same day.
9. Delete any duplicate events by selecting them and choosing Delete from the Edit menu. You may want to retain a single version of a duplicate event, but unwanted versions should be removed.

Note: If you have a large number of duplicates that would be overly time consuming to clean up by hand, you may want to use a third party utility such a Busycal, Calendar Cleaner, or iCal Dupe Deleter. For information about how to use these applications, refer to their built-in help or contact the vendor.

You can always try to update your device to the latest iOS version 11.1.1 by going to Settings > General > Software Update. If there's an update, tap Download and Install. If your iOS is already up to date, you see the latest version number and no option to install a new version.
Hope this information is helpful. Let me know if you need additional help.
Regards.