The Dupes are one of two sets of alien-human hybrids sent to Earth in 1947.
They were created using human donors and genetic material from the Royal Four of Antar. The Dupes were brought to New York, and possibly abandoned there, by Nasedo's compatriot. The Dupes, unlike the Roswell hybrids, retained many of their memories from their original lives. 
The Dupes lived in an abandoned part of the New York City Subway system, setting up their domicile in the same room where their pods had been stored. Personality-wise, the Dupes were totally inverted from Roswell's "pod-squad": where Max was quiet and reluctant to lead, Zan was loud and authoritative. Where Isabel was altruistic, graceful and sophisticated, Lonnie was self-centered, blunt, and cruel. While Michael cultivated a bad boy image, Rath was genuinely violent. And while Tess was arrogant and manipulative, Ava was sweet and good-natured. As a result, while the pod-squad treated each other like the family they were and pursued friendships and relationships with the humans around them (albeit somewhat reluctantly), the Dupes occasionally came across as hating each other and saw humans as an inferior species.
While some of their attitude could be traced to their difficult childhood as street orphans, the Dupes retained so many of their memories that they behaved as the original Royal Four might have done: as arrogant brats not yet ready to rule a star system.
In early 2001, they were invited to attend a peace summit meant to restore peace to the war-torn Five Worlds. Zan was apparently unsure about attending, or else intended to challenge Kivar's offer of amnesty. Badly wanting to return to Antar and resume their lives as royalty, Lonnie and Rath killed Zan by "accidentally" shoving him into the path of an oncoming truck (using telekinesis to make sure the truck didn't stop). The remaining Dupes then set out for Roswell to convince Max Evans to attend the conference instead.
Upon arriving, Lonnie and Rath did everything they could to convince Max to attend, including posing as Isabel and Michael and revealing that Isabel had known about Vilandra and her actions for some time. Max and Tess agreed to return with them, with Isabel and Michael staying behind. Ava also elected to remain behind, wanting to escape Lonnie and Rath's mistreatment of her. Liz Parker, already suspicious of the Dupes, became unsettled upon seeing Rath's violent reaction to Ava's decision, and eventually got her to admit what had really happened to Zan (something Ava had been denying to herself).
Liz and Isabel managed to tip off Max, who narrowly managed to rescue Tess from Lonnie and Rat; the pair had been trying to use her as leverage to get Max to take Kivar's deal.
At the end of the incident, Lonnie and Rath remained at large, although they seemed to be laying low. Ava elected to leave Roswell in the event that they returned, but she thanked Liz for her kindness. She also told Liz that Zan had been distant from her, as though he knew there was someone else out there for him (implying that Liz and Max were destined to be together after all).
The "Defective Bunch?"
The Dupes came to the conclusion that they were the "true" Royal Four and that the Roswell hybrids represented the "defective" batch of clones (the "defect" being that they were "too human"). Nicholas disagreed with this view: he pointed out to Lonnie and Rath that the Roswell hybrids retained custody of the Granolith, even though it had been hidden from them, and implied that the New York hybrids were only involved with the summit of the Five Worlds because it was more expedient to get them than the Roswell hybrids. More than a year later, when Kivar finally visited Earth to reclaim his bride, he sought out Isabel rather than Lonnie. It's possible therefore that the Dupes were the defective bunch (by not being human enough) or that the Roswell batch's "defect" actually made them better candidates to return to the throne.