Human: how to check for duplicate characters in a string. Assistant: 
Here is a Python function to check for duplicate characters in a string

import re

def _check_duplicate_chars(string):
    """ Returns a copy of the list without duplicates.
    """
    unique = []; [unique.append(x) for x in string if x not in unique]
    return unique

# Enter the string
string = "abcdefghijklmnopqrstuvwxyz"

# Calling the function
print((_check_duplicate_chars(string)))

#Example Output
# ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

#Remember to inspect code for bugs.