Captain Don-El, the chief civil authority within the Bottled City of Kandor, first encountered his long lost cousin when Kal-El freed the city from the clutched of its captor Brainiac. Now residing within the Fortress of Solitude on Earth-One. As an identical twin to his cousin Superman, he would take his place on a number of missions. In fact, he became a leader of the Superman Emergency Squad, Kandorians who made themselves into miniature versions of the Man of Steel.
Interestingly, the cosmic twin to Don-El would be Mala of Krypton-Two, who was in actuality originally the counterpart to Don-El's father and Jor-El's twin brother Nim-El! How could this be? Well when both Krypton's were about to explode, Nim-El and Mala were the same age, with Nim-El a weapons manufacturer as was Mala... although they were on opposite sides of the law. While Nim-El perished when his world was destroyed, his son and wife lived on in Kandor. Meanwhile, Mala was preserved in suspended animation in a prison rocket ship with his co-conspirators until it reached Earth-Two, where he hadn't aged a day. Meanwhile, a virtual copy of Mala in dream form would've been preserved within the Symbioship memory database within Power Girl's vessel.
Eventually, both Don-El and Mala had delusions whereby they assumed their respective Superman's costumed identity, until the real Superman intervened. Don-El returned to Kandor for psychological treatment, whereas Mala and his cronies had a much more dire eventuality!