Duplicate birthday in iPhone calendar list

Hi,

I googled 'duplicate birthday' issues, but I still can't solve this problem.

I have iPhone 4S (iOS6) only, so I donot use iCal.

My iPhone calendar shows only 1 birthday in a monthly view or daily view mode; however, it displays 2 birthdays in the list mode!

In the calendar options, it says I am reading iCloud (all), Facebook events (I don't have birthdays information in facebook), Holidays, and 1 birthday.

If I turn off birthday, then all of the are gone.

When I check in iCloud, it shows only 1 birthday, which is correct.

Could you please help me resolve this issue?

Cheers,