I agree with it being fine to use the same portrait or extant gown for inspiration, we're all using the same base resources! if it was just using the same fabric in a different style, totally have at. Same fabric and same style? I think you're probably find but if it was someone I actually knew and might wear it to the same event I'd probably shoot off a courtesy "hey FYI I'm going to make this, lets not show up in matching outfits" unless you WANT to show up in matching outfits and that's another issue entirely :-)

But for example Stephanie and Adrienne and I all have things from the same plaid, there's just not enough good fabric designs sometimes :-)

Now enough teasing what are you making!