What an interesting thread MamaC!
I have twins who are nearly 4.
I have always tried to ensure they are treated as individuals and in common with other posters dressed them differently and we never refer to them as "the twins". OTOH they are very proud of being twins and will usually tell people that they are on first introduction.
Thinking about it most people do send them seperate cards, which I display on seperate shelves. My little boy is not really that interested in cards but my daughter (far more interested in arts & crafts) likes to look at them & will cut them up for collages after they come down.
They occassionally get a joint gift (one of my friends takes the opportunity to buy them something special for their birthday that they can both play with but costs more than she could spend on them each) and they arw very happy with that. She always buy individual Christmas gifts though.
Interestingly although I generally send two gifts to a party I have only ever sent one card - I'd never really thought about it until this thread. I supose it's because I write the card. Next year when they can hopefully write their own cards and will be in different school classes (and I expect going to different parties) I'll buy two cards.
IME you have to gently educate your friends and family about what is appropriate for twins, you can't expect them to have the same understanding of the importance of establishing identities unless they happen to be close to other twins.
Let it go this year and maybe gently hint in advance next year.