find_first_dup

def find_1st_dup(array):
    found = {}
    for index, item in enumerate(array, start = 1):
        if item in found: return found[item]
        found[item] = index
    return -1

def find_1st_dup_value(array):
    found = set()
    for item in array:
        if item in found: return item
        found.add(item)
    return -1

tests = ([20, 10, 30, 50, 30, 20],
         [2, 4, 3, 5, 1],
         [5, 3, 4, 3, 6, 5],
         [400, 299, 170, 70, 40, 500, 60, 99, 299, 76])

for test in tests:
    print(f'index: {test} is {find_1st_dup(test)}')
    print(f'value: {test} is {find_1st_dup_value(test)}')