def find_aba(code):
    """Checks for existence of a 3-letter palindromic substring within `code`
    The palindromic substring must contain 2 unique characters
    """
    palindromes = []
    for i in range(len(code) - 3 + 1):
        # substring = code[i:i + 3]
        if code[i] == code[i + 2] and code[i] != code[i + 1]:
            palindrome = code[i:i + 3]
            palindromes.append(palindrome)

    return palindromes