Should keep the original precedence after dedup selectors with same filters #490

After the dedup of selectors with exact same key filter and label filter, the precedence of selectors should not change.

Consider case:

        options.Select("app.*", "dev");
        options.Select("app.*", "prod");
        options.Select("app.*", "dev");

Without dedup, "dev" wins. Dedup logic below will remove last occurrence, "prod" will win eventually. The dedup causes the change of precedence.