Birthdays for contacts are incorrectly duplicated in Calendar

Birthdays i have entered in Contacts appear on my calendar as expected. However recently i find that random dates are appearing with that same persons birthday information. This is happening on my iPhone and iPod.

For example correct birthday is say - May 10th and the entry is there. But also on say - april 24 -there is also a mention of that same person birthday . I do not have duplicate entries in my contacts. What is happening?

[Re-Titled by Moderator]