Merge arrays with duplicated elements

merge_lists.py

def merge_lists(aList, bList):
    if not aList or not bList:
        print("empty lists")
        return aList + bList

    c = []
    while aList and bList:
        if aList[0] < bList[0]:
            c.append(aList.pop(0))
        else:
            c.append(bList.pop(0))

    return c + aList + bList


def main():
    lista1 = [0,3,7,9,10]
    lista2 = [-1, 0, 3, 18, 20]
    print(lista1, lista2)
    result = merge_lists(lista1, lista2)
    print(result)


if __name__ == '__main__':
    main()