C, the new head of a certain intelligence agency, is aware that there is an impostor hiding among the ranks.
In order to find the impostor, he has decided to ask all field agents to immediately link into a secure line, in order for C to preprocess them all.
The field agents call in by their call signs, as follows:
After compiling the list of agents, C notices an agent who cannot possibly exist, and immediately orders his arrest.
Who did C find?