If I want to ask a question that has already been asked,but I can't understand its solution.Should I comment there for more explanation or ask mine by adding duplicate by myself.
Just my opinion, and definitely not an absolute one.
It depends on the age of the original question and how extensive the needed additional explanation is, but i'd say if the question is not "fresh" and you need a considerable amount of additional information, I'd ask a new question, referring to the older one, and clearly describe why you don't understand the answer on the existing question.
Asking additional explanation in a comment on an old question might be unnoticed and unlikely to have any effect on the answer. In a way it's a new question then about the answer of another one :)