Duplication abounds in Greek mythology. The story of the Gigantomachy, a sequel to the similar Titanomachy, is an excellent example.
After the Titanomachy, where Zeus defeats the Titans and places them in Tartarus, an angry Gaia brings forth the giants (gigantes) to restore the rule of the Titans. These giants are not simply big, but have multiple hands and snake legs. Pindar mentions the theft of the cattle of the sun god Helios, as initiating combat. Another version makes the precipitating factor the rape of Hera by a giant named Eurymedon (or Porphyrion). The gods, led by Zeus -- now more powerful than in the Titanomachy because he has thunderbolts, fight against immortal giants, led by Alcyoneus the cattle-thief and Porphyrion. The result is inevitable: Zeus wins, but only with help from Hercules / Heracles, Alcyoneus's killer. Alcyoneus is only immortal as long as he is in his place of birth, so Hercules removes him from Phlegra. Hercules does this elsewhere in Greek mythology when he lifts the Libyan giant Antaeus to remove him from the source of his strength, his mother, Gaia.
Another duplication in the Gigantomachy is the prophesied need for 2 demigods to defeat the giants. The second son of Zeus is Dionysus, who, accompanied by satyrs and silenoi, kills Ephialtes with his thyrsos. After the rest of the giants are killed by the gods, Hercules shoots them with his bow -- just to make sure they really are finished.
After their defeat, and in contrast with the version of them being killed, the giants are placed beneath the Earth where they continue to make mischief by causing volcanoes and earthquakes.
See #5 on Thursday's -cide words to learn.
Battle of the Giants.
Source: Michael Grant From Alexander to Cleopatra.