Used to express astonishment when somebody says or does something completely out of character. The only possible explanation is that they've been replaced by an alien shape-shifter who looks exactly like them, but acts differently.
The phrase is a parody of the cliché found in old movies, when somebody returns home to find their lover gone and a stranger in their place.
Shanee: "I'm going to stay in tonight and study."
Ruthie: "Who are you, and what have you done with Shanee?"
An internet meme which is often used to describe something extraordinary, commonly a rare creativity which has a relation to science as a whole.
Person A: Hey, look at this: *link*
Person B: What is it?
Person A: Look for yourself, I call it the multitude-bathduck-replicator Person B: WHAT HAS SCIENCE DONE?
An expression used to say that one has been in the situation or experienced that which another is talking about. It is usually a negative scenario that is being discussed.