Are all kilograms created equal?

I was playing a game with some friends and hanging around on Discord last night, one of my friends and me started quoting Limmy bits when the "kilogram of steel or kilogram of feathers" bit came up. One of my friends who I generally consider to be intelligent piped up and said he recently learned that there are certain materials where a kilogram is heavier than a kilogram of another material. At first I was convinced he was fucking with me but he was so serious that I need a sanity check, I'm in IT and any physics class is faaar behind me, is my friend as dumb as I think? Or is this actually a thing?