Python

O(n):

def duplicate_value(str_arr):
    """A duplicate must appear in str_arr"""
    tally = dict()
    for c in str_arr:
        tally[c] = tally.get(c, 0) + 1
        if tally[c] == 2:
            return c


assert duplicate_value(['a','b','z','x','y','z']) == 'z'
assert duplicate_value(['z','b','c','x','y','z']) == 'z'
assert duplicate_value(['a','z','z','x','y','z']) == 'z'