Previously we considered two nuclear powered heroes and their flame-haired girlfriends. Our next several candidates for consideration are a cadres of crooked costumed crusaders who, through phenomenal physiques and careful coordinate their perfect replicas of themselves proved threats to our dynamic duo. 
Danton Black was Professor Martin Stein's assistant at the Hudson Nuclear Facility, and when Black's jealous meddling initiated a set of circumstances ultimately leading to a nuclear meltdown, not only did Stein and young Ronnie Raymond become Firestorm... Black become something more as well. As opposed to Stein and Raymond's fusion into a combined being, Black's power was fission leading to the creation of similar smaller duplicate. Each possessed super-strength and tough exteriors, along with a mental connection with Black himself. Donning the costume of Multiplex, Black repeatedly attempted to extract revenge on Firestorm.
His counterpart on Earth-Two was the Leader of the Secret City, a secluded civilization within the Amazonian jungle. There is evidence that this is a 20th century copy of the 12th century villain known as Thando, who battled the Silent Knight a hemisphere away in England. Thando was able to self-replicate, a skill mastered by his identical twin the Secret City Leader.When a journalist disccoved this city, the inhabitants kept him imprisond until he himself mastered the ability that the leader had of creating a duplicate. He was able to bring the Flash and Doctor Maria Flura there, with the Flash defeating the Leader.
While it was Multiplex's machinations that caused Lorraine Reilly to be kidnapped and experimented on, leading to her becoming Firebrand, Hourman's relationship with Lorraine's counterpart Danielle was romantic albeit brief. Yet both involved the gals' father, Lorraine's U.S. Senator father and Danielle's steel magnate, in failed attempts by criminals to broker deals with their sires it led to the wrath of their daughters. This was the end of both relationships.
As for the Soldiers of the Secret City, versions of them returned with scientist Professor Lexon colloberated with Flura in developing a device that replicated the replication powers. Once he defeated these soldiers, the Flash uncovered that it was Professor Lexon... not the Leader himself.... who was the criminal culprit.
