For example, if someone is given to finish two contradictory things (or we can say task) and he/she would accomplish all at the same time. It's like oxymoron, but somebody can handle those kind of situation. Is there any word for that?
You could say the completion of both those things (or tasks) as a whole is dichotomous. According to Cambridge Dictionary, dichotomous means:
"involving two completely opposing ideas or things"1
Some examples from ODO:
"‘Crewdson's work is replete with dichotomous tensions.’" 2
"‘This view emphasizes the dichotomous thinking and behavior often displayed by alcoholics.’" 2
"‘The forced-choice format is used with each item presented separately and each pole representing a dichotomous extreme for one attitude or function.’" 2
According to ODO, a dichotomist is:
"A person who dichotomizes or classifies by dichotomy."
So in your case you could the person (or phenomenon) you described a dichotomist. I should add that the word isn't commonly used (only 18,000 hits on Google). It is, however, a proper word and has been used since the 1860s (according to this ngram).
1 Dichotomous Meaning in the Cambridge English Dictionary. (n.d.). Retrieved March 23, 2018, from https://dictionary.cambridge.org/dictionary/english/dichotomous
2 Dichotomous | Definition of dichotomous in English by Oxford Dictionaries. (n.d.). Retrieved March 23, 2018, from https://en.oxforddictionaries.com/definition/dichotomous
3 Dichotomist | Definition of dichotomist in English by Oxford Dictionaries. (n.d.). Retrieved March 23, 2018, from https://en.oxforddictionaries.com/definition/dichotomist