If you have the unwavering belief a loved one has been replaced with an impostor (and you don't have the pimp mustache) you're probably dealing with Capgras Delusion. Besides being the name of band you were in while at community college, Capgras Delusion is brought on by that Molotov cocktail of brain disorders: schizophrenia.
The alleged impostor can sometimes be perceived as a threat or simply as a docile stranger who just happens to look exactly like your spouse and inexplicably wants to live their life. While most Capgras patients decide to live with the impostor, some rebel against the perceived takeover and try to fight it. They can become so paranoid that they even believe they themselves have been "replaced."
The question is, if you think you've been replaced, why were you not replaced with a fitter, more well hung version of yourself? We mean, as long there is a pretend "you" running around, that pretend "you" might as well get pretend laid more often.