Comment by Joker_vD

The SSHD internally has a config structure. Initially, it's all initialized with -1 for flags/numbers/timeouts, and with NULL for strings. When an option is parsed, there is a check whether the config structure's corresponding field is -1/NULL (depending on the type) before the parsed value is saved.

Another program with first-wins I've seen used dict/map for its config so the check was even simpler: "if optname not in config: config[optname] = parsed_value".