Remove duplicate item from an Array

I am working on a project where there are 10 questions and user can answer the question or skip the questions. I want to track the number of questions (which questions) the users have skipped. I am adding the skipped questions in an Array. But if users click on the previous button and skips the same question again, the same question number is added twice to the skipped question Array. Is there any solution so that I could delete the duplicate entry in an Array.
My Array looks like:
I simply want to trip the duplicate entry and get the output like
Please let me know if I am not clear