birthdays appear twice in my calendar

Another post asked why birthdays appear twice. That one failed to answer the question, and the discussion was closed.

Here's the answer. The most likely source is duplicate entries in your Contacts. Sometimes after syncing your iPhone or iPad the entire contact list will be duplicated. Since the calendar draws birthday information from the Contacts the birthdays will be listed twice in the Calendar.

To solve the problem go to Contacts>Card>Look for Duplicates. Chose Merge. The duplicate entries will disappear. You may have to close and reopen the Calendar to eliminate the duplicate birthdays.