Combining Lists

I have two lists such that each list contains no repeated elements. However, the lists do share some elements, and I want to combine the lists such that no elements are repeated. For example, if list_1 is $[1,2,4]$ and list_2 is $[1,3,5]$ , then the combined list should be $[1,2,4,3,5],$ not $[1,2,4,1,3,5].$

To accomplish this, what should go in place of "FILL ME IN" in the code below?

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for element in list_2:
    if "FILL ME IN":
        list_1.append(element)
print list_1