In Antigone, what does the following quotation mean: "robbed of two brothers both dying the same day by doubled hand"?

The quotation you cite refers to the deaths of Polynices and Eteocles.  Antigone and her sister Ismene have been "robbed" of their two brothers because the two went to war over the rulership of Thebes.  When Oedipus exiled himself and eventually died, it was decided that Eteocles and Polynices would take turns ruling Thebes, one brother taking a year of rulership and then reverting to his brother.  However, after the first year is over, Eteocles does not want to relinquish the rule of Thebes to Polynices, so Polynices wages war on his brother and on Thebes.  Polynices puts together a makeshift army, and they storm the city gates.  Polynices and Eteocles end up in battle with each other, and they kill each other simultaneously by sword (in other words, the "double hand").