When you find someone doing porn who looks just like a friend, but said friend insists it isnt her rubbing one out on her webcam (though it probably really is)
"Hey is this (insert whatever porn website mysteriously popped up on your computer) you"
"Are you sure?"
"Yes! It must be my dopplebanger. Why are you looking at asian porn?"
"Um.... got to, I have a phone call. fapfapfapfapfap