Can we use "ещë" for repetition, as to say "he came once again today", or "they once again received as strange letter", or only for continuity? (= for ex: "It's 10 in the night, but she's still working")
Yes, "ещë" can be used for repetition, but usually in form of "ещë раз".
"he came once again today" can be translated as
Сегодня он пришел еще раз
There is better translation though: "Сегодня он снова приходил"
"they once again received a strange letter" can be translated as
Они получили странное письмо еще раз
Better translation is "Они опять получили странное письмо"