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

I don't think it would be plagiarism to ask similar or same questions.. Unless the question is a direct copy and that's not possible because musing wouldn't allow it.

There could be some parameters to why this can happen. One would be you didn't use musings features properly.. Before asking questions just do a short search using musings own search engine and if you find a similar question then see if it has been answered. if it is then why bother asking another similar and waste your precious time. But if you dont find any then it could be because of two reason. One would be the other questions has been asked using different interrogative syntax. Or no one has asked it yet and you are free to ask..

Now to come back to the plagiarism criteria, a short Google search will tell you what plagiarism actually is.

Definition of Plagiarism is "the practice of taking someone else's work or ideas and passing them off as one's own."

Questions are not ideas. They come before the ideas are even generated. its the answers that if copied could be taken as plagiarism. So dont be afraid to ask anything.. Hope this helps you in any way.