my brother and sister are twins and they were born three days before my 2nd birthday.
they always got shared cards, but not shared prezzies. I always had to share my party with them, usually on the saturday between the birthdays IYSWIM. My elder sisters birthday was 9 days after mine and she always got her own party 
When my little sis was 30, she had a 30th party, quite close but not on her birthday, in her new house. I went and while i was there i wished bro happy birthday and she went nuts!
'cannot wish him happy birthday, its not his birthday, you have no idea how horrible it is sharing every party' she looked exactly 
(Actually I do)
'you singletons get it all!'
this was aimed at my DP
thing is, no only do i know what its like, so does my DP who was actually born on his sisters birthday, so had to share everything too. They are 3 years appart and they used to get shared cards too!