I thought I'd post as this popped into my brain but I always remembered this from my 7th birthday party because even then I thought it was unusual!
I was friends with a twin, let's call her Pam. Pam had a twin brother, let's call him Mick. I was friends with Pam but not Mick. I invited Pam to my small party at home for my birthday, along with a couple of other girls in my class. On the day, Pam was ill, so their mum sent Mick instead of Pam. Was this a bit strange of the mum? Should I have expected to have Mick there too as they are twins?
Obviously, this was so long ago so doesn't matter 😂 but it's always stuck in my brain.