The Boy Who Knew Her Secret was going to be developed into a Wonder Lad origin story, but that idea was scrapped.
On home video, this episode is missing the "previously on" recap at the start. It starts with the opening credits, rather than the cold opening recap, which results in a significantly shorter runtime.
The subject of impostors is covered several times in this series. In "The Boy Who Knew Her Secret" an alien comes to earth who impersonates other humans and animals. In "Mind Stealers from Outer Space" aliens called The Skrill come to earth who inhabit humans and take over their bodies. In "The Last of the Two Dollar Bills" Nazi spies who are modified to look like US citizens infiltrate a cafe in the Washington DC area. In "Fausta The Nazi Wonder Woman" there is a lookalike Wonder Woman who takes her place for awhile. In "The Deadly Toys" a toy-maker creates doppelgangers which take other people's places in the story. A Nazi Spy dresses up like Steve in "The Feminum Mystique" and attempts to take his place on a plane; before the US forces and Wonder Woman catch him. Also several times throughout the series Wonder Woman impersonates other characters by mimicking their voices.
The Boy Who Knew Her Secret (Parts 1 and 2) and The Mind Stealers From Outer Space (Parts 1 and 2) have similar plots. Both involve alien imposters trying to take over the planet; in Mind Stealers the aliens put an egg on human's foreheads and are able to jump into their body and take over; in The Boy Who Knew Her Secret the aliens are simply shape shifters who take the place of people. In both episodes also someone learns of Wonder Woman's secret identity.