This gem of an episode from my childhood is fun to watch, barring the anticlimactic and rather silly coda to the occupation of Paradise Island by the Nazis. For spies, they aren't very observant. Captain Radl noticed that Drusilla wasn't as tall as he thought Wonder Woman was after capturing Dru in part 1, yet never considered they had the wrong girl, despite the fact that Dru's costume is very different from Diana's.
The detail continues to slip past Radl on Paradise Island as Dru, in her Wonder Girl costume, participates in the plotting and rebelling even with the real Wonder Woman standing right there. No one realizes there are two females in two unique costumes.
Peter Knight, having dined with Dru once, does not recognize her when they are imprisoned together, thinking she is Wonder Woman. Dru doesn't look or act much differently while in costume and Knight is even reminded of her when considering how to manipulate her into giving him information about the bracelets. Another thought, since Dru, in her guise as Diana Prince's sister, claimed to have been kidnapped by the Nazis and held captive with Peter Knight whom she identified as a spy, would she be a necessary witness at Knight's treason trial? If so, there could be complications there as Knight thinks he was imprisoned with Wonder Woman!