Transporter duplicate
A transporter duplicate or transporter clone resulted when a transporter accident created two copies of the same person or object. Refraction or reflection of the transporter beam was usually to blame for such duplication. 

The duplication process could sometimes result in two incomplete copies, as was the case when both a canine species and James T. Kirk were duplicated in 2266 after a strange ore from the planet Alfa 177 altered the transporter's function. Though physically identical, each copy lacked certain personality traits of the original. One canine was docile, while the other was angry. Testing of the re-integration process on it was successful materialization-wise; however, the creature died as a result of emotional shock.

In Kirk's case, duplication created two distinctly different individuals, a "good" Kirk (lacking willpower and command strength) and an "evil" Kirk (violent and self-indulgent); neither copy could function as a complete person. The transporter was successfully used to re-merge the incomplete copies into one functioning individual, reversing the original duplication. ()

The duplicates could also be complete copies of the original, not only physically identical but with the original's personalities and memories intact. In 2361, then-Lieutenant William T. Riker was duplicated when the containment beam for Riker's transporter signal on Nervala IV weakened due to the planet's distortion field. To compensate, the transporter chief aboard the  created a second containment beam, with the intention of integrating the two. 

However, Riker's transporter signal made it back without help from the second beam, which was reflected back to the surface, creating an exact copy, Thomas Riker, who was rescued eight years later by the . Unlike the two Kirks, the two Rikers were initially identical but became different individuals as a result of their different experiences. ()

In 2371, Gul Dukat, upon being informed that Maquis member Thomas Riker had hijacked the , assured Benjamin Sisko that the Cardassian Central Command would not "believe [his] story about transporter duplicates and security failures"; instead, they would assume that Starfleet gave the Defiant to the Maquis, and respond to the threat with a full-on assault. ()

In 2381, Lieutenant junior grade Bradward Boimler, serving on the , participated in a covert mission on Karzill IV, which was dicovered by its Pakled targets.  Due to a distortion field, the team could not be beamed out.  Boimler used his knowledge of the previous incident involving Riker, his captain at the time, to enable the Titan to lock onto their signals despite the interference, saving his three comrades.  When Boimler's signal was nearly lost, a second containment beam was initiated and Boimler was successfully beamed aboard; but he was also duplicated and the one still on the ground escaped via shuttlecraft moments later. Upon reboarding the Titan, the two Boimlers confronted each other and were ultimately brought to Riker's ready room. Riker informed them that Starfleet forbids clones from working on the same ship and one of them would have to go back to Boimler's previous posting, the , with a demotion to Ensign. By this time, the two Boimlers had become sufficiently differentiated that only one of them volunteered, and expressed surprise that the other did not.  The volunteer continued to be known as Brad Boimler, while the other takes up the name William Boimler ()

Because the two individuals created by transporter duplication are identical to the person who existed prior to beaming, the term "transporter clone" could apply to either of them; Brad Boimler described Thomas Riker as "[William] Riker's transporter clone," but the successfully beamed Boimler also referred to himself as "the transporter clone" in that situation. ()

Lifeform classifications
Transporter technology