Advent of code - Day 3 - First task

alphabet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
duplicate_chars = []
total_sum = 0

with open("text", "r") as input_data:
    for line in input_data:
        half = len(line) // 2
        first_half = line[:half]
        second_half = line[half:]

        temporary_chars = []
        for ch in first_half:
            if ch in second_half and ch not in temporary_chars:
                temporary_chars.append(ch)

        duplicate_chars.extend(temporary_chars)

for ch in duplicate_chars:
    total_sum += alphabet.index(ch) + 1

print(total_sum)