如何快速防止对象ArrayList中的重复

var firstArray = [1,2,3,4,5,6]
var secondArray = [1,2,3,7,8,9,0]

for item in secondArray {
//    some thing that i can avoid duplicate
    firstArray.append(item)
}
print(firstArray)
//end result should be firstArray = [1,2,3,4,5,6,7,8,9,0]

我想防止firstArray追加重复项