ME thinks the secret service got so good at telling the difference between a real U.S. Dollar and a counterfeit one, they were appropriated to tell the difference between a doppelganger fake president or someone in make up, and the real president.
So they had to follow the "real" president everywhere to protect him from being doubled.
I could see plots and foils to grab power by bringing in a double.
People double as Q, happens here all the time.
It happens to celebrity all the time, people double for celebrity's to make appearances and make money, some get caught, and some don't.