There are several moments during the Kirk-against-Kirk fight when Shatner's double's face is visible and it clearly isn't William Shatner.
Although Spock was never hit over the head by Garth (at Leonard Nimoy's insistence during filming), they forgot to change the script - at the end Kirk says, "Mr. Spock, letting yourself be hit on the head..." Spock was never hit over the head.
There are several moments during the Kirk-against-Kirk fight when Shatner's double's face is visible and it clearly isn't William Shatner.
Instead of all of the inmates of the asylum wearing those ridiculous costumes, wouldn't it make more sense for all of them to be wearing some sort of standardized hospital clothing?
Mr. Spock
is confronted with a choice between Captain Kirk and Garth, masquerading
as Kirk. Chief O'Brien would face a similar conundrum in "Star
Trek: Deep Space Nine"'s third season finale "The
Adversary", when he must choose between Odo and a changeling
masquerading as Odo.