The belief that a loved one has been replaced by an imposter is called Capgras delusion. It was first identified in 1923, when a psychiatric patient claimed her husband and children were "the object of substitutions."
One of the most interesting aspects of Capgras delusion is the element of love. Usually it is not a stranger who has been replaced, but a beloved. In the 1990s, psychologist Haydn Ellis and others theorized that Capgras delusion is the result of your mind recognizing a face without feeling the love that you normally associate with that face.
Love is how we know we are in the right time.