It's happened before in real life.
Here is a 1994 article describing an Illinois criminal trial where defense counsel pulled the old switcheroo and sat a different person with him at the defense table instead of the defendant. The defendant, instead, sat somewhere else in the courtroom.
After a witness misidentified the perp as the person at the defense ...
It is illegal to advertise yourself as a "Private Investigator." However, businesses under the name of "흥신소(background checkers)" is legal and their scope of operation is hard to distinguish from that of Private Investigators.
Therefore, it is not illegal to hire someone to collect evidence, interview witnesses as long as those actives are within the bounds ...
Competent evidence is admissible under the relevant rules of evidence for the tribunal.
Material evidence bears directly on a point that is in contention before the tribunal.
Substantial evidence is compelling in deciding the point.