I have an identical twin. We have shared partners a couple of times and I think our cocks are very similar looking when erect. I do remember one person stating that my dick looked more "steely" but that was probably due to the level of excitement that each of us was feeling at that moment. Soft, I think our cocks are fairly similar looking, though I like to think mine hangs a little longer than his does when soft. LOL
I remember one time shortly after we both had reached puberty and I walked into our room, he was laying in bed with erection in hand (presumably he had been masturbating), and he had a pretty good amount of "pre-cum" coming out of his dick. Since I had never seen more a drop of two from mine I was curious about why his cock looked so wet and shiny. A year or two later I asked him about his pre-cum, and apparently he "makes" a quite a lot when masturbating or aroused, while I am nearly dry.
We have always been very competitive and play sports like tennis and racquetball against each other, so we have seen each naked in the locker room and showers most of our lives. We are very comfortable about being naked around each other. Our bellybuttons are different (his is more of an innie while mine is a bit "swirly") and that has always been the case as far back as I remember. As for the rest of our bodies, we are pretty identical weighing within 5 lbs of each other, having the same shoe size, shirt size, etc. He has a very small amount of hair around his nipples and I don't. I have a more prominent "happy trail" then he does. Anyway, hope that answers the OP's questions.