Miles Edward O'Brien was a replicant, or clone of the original Miles O'Brien in the Star Trek: Deep Space Nine episode Whispers.
Like the original O'Brien, the replicant O'Brien was portrayed by Colm Meaney.
O'Brien was created by the Paradan government in 2370 after the government of Parada abducted O'Brien, who had traveled to that world to help the government and rebels prepare for peace talks. The Paradans were experts in cloning and able to very precisely duplicate individuals, right down to perfectly duplicating all memories and personalities of individuals so cloned.
This O'Brien was sent back to Deep Space Nine believing he was the real O'Brien, and was intended to be activated during the peace talks for some nefarious purpose during the peace talks between the Paradans. Most likely this would have been the assassination of one or more members of the rebel peace delegation.
Before O'Brien returned to DS9, the Paradan rebels learned that the real O'Brien had been captured, and the replicant O'Brien had been sent bac to DS9 in his place. They tipped off Commander Benjamin Sisko to what had happened. Doctor Julian Bashir performed a very through medical exam but could not prove that O'Brien was a replicant. The crew decided to lock O'Brien out of everything and keep him from the talks until they could be sure. Even O'Brien's wife and daughter Molly were behaving in a way that set alarm bells off in O'Brien's head.
When Odo returned to the station O'Brien told him what was going on. Odo at first believed a conspiracy was afoot and told O'Brien he would investigate but was soon pulled aside by Sisko and Major Kira who filled him in on the situation. DS9 tried to place O'Brien under arrest but he managed to escape in a runabout. Reaching a Starfleet Admiral (who unknown to O'Brien had also been briefed on the situation) he was told to surrender to the station, leading O'Brien to think all of Starfleet had been compromised.
Feeling he had no choice, O'Brien took off for the Paradan homeworld, intending to warn the government that had created him that Starfleet and the Paradan rebels had been conspiring against him. Sisko, Kira, and Bashir took off in pursuit but broke off when they were informed the real O'Brien had been rescued and was safely with the rebels.
Beaming to the rebel installation where Sisko, Bashir, and Kira were treating the real O'Brien, the replicant O'Brien beamed down to confront them. O'Brien was shot by an overzealous rebel with a weapon set to kill. As O'Brien laid dying the real O'Brien remarked that the replicant was a perfect copy of him. The rebels and DS9 crew realized the Paradans had made the replicant too perfect, that O'Brien's sense of duty drove him to escape DS9 and warn the Paradan government of what he believed to be a conspiracy against them. With his final breaths, the replicant O'Brien asked the real O'Brien to take a message home to Keiko that he loved her.