I got rid of 1 of everything as we no longer need them. We have boy/girl twins so I kept all their clothes. We're still using double stroller but I'll eventually sell it and get a single. We don't plan to start trying for #3 until the twins are 3 or so.
I've tried to sell (or more frequently put in the "to sell" pile) the clothes that might get me something in resale value. The other clothes I've split between a "Save" pile and a "donate or give away" pile. I don't want to bet on having multiples again because that would be kind of a worst nightmare situation for us. But I know with any 3rd child I'll want to have some stuff to reuse and want to buy some new stuff so I need like 50% of what one kid needs, not the 150% of what two kids need that I have.
We didn't really have duplicates of much stuff. We had two bouncers but they were both borrowed and have been given back. We had two boppies and have sent one on to my nephew. We also gave an infant car seat to the same nephew. That's about it so far. I guess when they're ready to move out of highchairs and a stroller we'll probably get rid of the double stroller and one highchair.
We got rid of the double snap n go and 1 of the duplicates- car seat, highchair, bumbo, twin nursing pillow, and stuff like that. Since we aren't finding out the sex, I saved all the clothes and have washed all the more gender neutral things. If it's a girl, I'll go through all the old clothes and sell most of it asap. We had identicals though, so I was nervous for twins again, but fairly confident lightening wouldn't strike twice.