A Doppelganger is a fictitious or folklore- someone who looks like someone else. It's become more popular recently because of the show The Vampire Diaries on CW- no explanation needed if you watch the show and too much of an explanation if you don't. :) BUT, here's a picture:
So, long story short, same actress plays 3 characters throughout series- Elena, the main character is on the left and her two doppelgangers Tatia and Katherine on the right. This is easily done with a quick hair style change!
Here are a few examples of doppelganger images off of Google:
Recently, I came upon this picture of my 1st cousin, 3x removed on my dad's side.
Samuel Anthony Trick
20 Feb 1889 Dayton, Ohio
16 Sep 1958 Dayton, Ohio
Samuel was married to Marie Margaret Brinkman (1887-1936), (whom I'll write about later in a different post- 52 Ancestors in 52 Weeks).
Samuel, in my opinion looks just like this actor:
James Paul Marsden
18 Sep 1973 Stillwater, Oklahoma
(Also known as the wonderfully handsome war hero in The Notebook)
Here's a pic of them side-by-side: