The double.

The doppelgänger is bad news. This double of yourself is out to kill you and lurks by your side your whole life, waiting for you to catch a glimpse so it can finally strike. A dream of a doppelgänger is a harbinger that you will see it in waking life, and spotting one for real is a harbinger that you will soon die. Just ask Percy Shelly, Goethe, or this guy. A doppelgänger is not a twin. We at least know where those come from. It's also not a clone, no science or laboratories involved here. Be suspicious of people who look just like you, or even kind of like you (a good resource here to help you avoid potential danger).