Find duplicate values in a String.
Return each duplicated alphabetic value once, ordered by when its duplicate is encountered. Lowercase letters establish values, repeated lowercase letters count as duplicates, and an uppercase letter counts as a duplicate only when its lowercase value appeared earlier. Ignore non-alphabetic characters.
Input: one string s. Output: a list of lowercase strings.
Example 1: s = "programming" returns ["r", "m", "g"].
Example 2: s = "Sears QA" returns ["a"], because a appears before A; the initial uppercase S does not establish a lowercase value.
Constraints: 0 <= len(s) <= 10^5; characters may be letters, spaces, or punctuation.
def find_duplicate_values(s):