Deeta Tarrant was the brother of Del Tarrant. While the two men were virtually identical (possibly due to their being played by the same actor), Deeta referred to Del as his 'little brother' shortly before his death, so they may not have actually been twins.
Deeta left Earth somewhat earlier than Del and eventually came to be champion for the United Planets of Teal. He was Champion for about four years, apparently defeating many challengers.
At some point his brother apparently learnt about this, since Del immediately set a course for the scene of the combat on hearing of war being declared. As a result of this declaration, Deeta went into combat against the Vandor champion, Vinni, and died when Vinni beat him on the draw. He had earlier passed up a chance to shoot Vinni in the back.
Although his brother was in the vicinity, they never met (Deeta had intended to see Del after the combat), but Deeta managed to transmit a dying message to Del through the sensornet implanted in his brain.