What if I ask question to musing and I didn't know that it was already asked by someone does it consider as plagiarism?

Let's say I went on musing.io and posted the question: Who do you think will win the Nobel Prize in Physics this year?

By "this year" the question is referring to the current year. Somebody could have posted that exact same question last year but common sense dictates that even though the two questions are worded exactly the same they are two different questions because they refer to two different outcomes. Likewise my posting a question about this year's Nobel Prize shouldn't prevent someone from asking the same question next year.

From a different perspective: Sometimes copyright infringement is done with innocent intent. "Innocent infringement" is a bad copyright defense. However, for copyright law to protect a work the work must meet a "threshold of originality" which I don't think the questions on musing.io meet. The descriptions to the questions might. As long as your descriptions aren't word-for-word the same as another question in my opinion you or anybody else should be fine innocently asking the same question.