It’s very different having twins after a singleton, vs having twins first. You have a toddler to deal with, which necessitates different activities. More relevant to this discussion, though, is that you already have some things and you’ve developed preferences for what you actually use.
Here goes on the double question (after a singleton). Some of this will depend on your preferences.
I need 2 of: car seat, crib, pack n play, high chair, bouncy seat. Most of these are safety issues so they can’t really be debated. I think most people would like a second bouncy seat, vs other locations for infants, but there is some personal preference here.
Need more of, but not double: bibs, crib sheets (I went from 4 total to 6 total), bottles (personally I like to only have to wash once per day, but that’s up to you)
Nice to have with twins that I didn’t have with one: swing (one is enough as your child might not like it anyway but they’re great when there’s one adult and they’re both crying), two bumbos. Not everybody likes the bumbos, but I found them invaluable for storytime with three. I could set them up in front of me like you see at library storytime vs trying to cram them all on my lap. That never worked very well.
Things you likely have that you don’t need more of: activity mat, exersaucer/jumper, changing pad covers (assuming you have more than 2 already), carrier so you can carry one and put the other in the stroller/shopping cart/etc (unless both adults are willing to carry).
You’ll also need a double stroller, or triple if your kids are too close together. I’d vote for the double if at all possible. If necessary you can put one twin and the older one in the stroller and carry the twin. The older one will learn to walk soon enough and triple strollers are even more expensive and bulky than doubles.