AFTER REMOVING DUPLICATE CONTACTS....

Some birthdays still appear twice in my calendar. Older answers recommend removing the duplicate contacts, but I've already done that and yet duplicate birthdays continue to appear on Calendar, but don't show in Contacts list. Now, in one instance, one contact had been listed as "Mike" and the other for the same person was listed as "Michael". After the merged duplicate contacts function was done, both still show up in the calendar, even though there is only one of this person in Contacts. If it were just one entry, I could simply remove and reenter that contact, but it is many and no doubt this occurred from when I merged my old Contacts from an old Android phone to my iPhone, or from when I imported Contacts from another email program to my Apple account. Hoping someone knows how to eliminate these duplicates completely and forever! Thanks.