An interesting anomaly took place between Earths One and Two involving a set of blonde bombshells known for their vibrating vocal cords. In Wonder Woman #288 through 290, we're introduced to Helen Alexandros who is an aspring ballet dance limited by only one thing... her appearance. Though talented, her "ugly duckling" face fails to land her an roles on stage. Granted the ability to transform from the homely brunette into the beautiful blonde known as Silver Swan, she only needs destroy Wonder Woman of Earth-One. Gaining the power of flight, great strength and a destructive swan song, she sets out to remove the Amazing Amazon from the equation. Along the way she becomes anamored by the gorgeous Captain Wonder, himself a mere fabrication of the sinister Doctor Psycho, and is ultimately defeated.
Thereafter, in Wonder Woman #291-293, the second Black Canary... Dinah Laurel Lance daughter of the original... returns to her homeworld of Earth-Two to assist Power Girl and the Huntress deal with an alien horseman known as Plague sent by the cosmic Adjudicator. Herself granted with a canary cry jsut as destructive as her doppleganger's swan song, she makes up for her lack of strength with her savy martial arts techniques. Once this case wraps up, Canary returns to Earth-One. But how can this be... with two counterparts existing on the same Earth simultaneously, leaving a void on its parallel planet?
Actually on Earth-One, the Steve Trevor that existed around this time wasn't that world's native Trevor, but a version from a parallel planet. The original Steve on that world died years before, so through cosmic happenstance he was replaced by another version. This compensated for the extremely long-lived Steve Trevor of Earth-Two. Similarly, while Earth-Two's Black Canary lived for years on Earth-One, assuming the place of Silver Swan who perished shortly after her debut.
Cosmic balancing is complicated, without a doubt, but always can be reconciled within the Multiverse!