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JavaScript

var arr = ["AAA", "BBB", "CCC", "AAA", "BBB", "CCC", "AAA", "BBB", "CCC", "DDD"];
var firstElem = arr[0];
var secondIndex = arr.indexOf(firstElem, 1);

// Output: ["AAA", "BBB", "CCC"]
var newArr = (secondIndex != -1) ? arr.slice(0, secondIndex) : arr;

alert(newArr);