I have a DS, he's been challenging, he has additional needs. But we are now expecting surprise twins at Christmas, and honestly, people have no idea of the difference.
Ours are boy girl. So minimal sharing of everything. Twice the bottles. Half the sleep. Twice the nappies, wipes, two cots, and leading on to the requirement for two separate bedrooms as they grow. We didn't ask for twins. We asked for one. So we should have simply aborted both and lost our "one" because it turned out to be "two"?? 
So with our requirement for extra bedrooms, we need a new house. Oh, and the two extra car seats and double twin suitable buggy means a new car. Because we got a surprise extra child, we need a new house and a new car. We had no choice about the extra. And we need a new house and car as a result.
Sibling discount for childcare? Try 5%. May as well not bother.
Double the birthday expenses. Double the school uniforms. Double the food and double the holidays. Double the swimming lessons. Double the first bikes. And all of these doubles aren't with the "luxury" of being staggered over 2 or more years. They're all payable together. Finding £200 today and finding £200 again in two years time, is not the same as having to scrape together £400 in one go today.
And what extra support do we get in recognition that we have a whole extra person turn up that we did not ask for, simultaneously with the other baby? That we've got double the costs and half the time? Sod all.