If you write something from your own mind it is very unlikely that anyone else would have written the exact same thing before you. If your work is unique then it really can't be duplicate.
The duplicate notice comes up for one of 3 reasons:
1. the work is copied from another website.
2. your article has a lot of quotes in it e.g. from Shakespear or the Bible.
3. someone else has copied your work and posted it somewhere else on the internet. In this case, your article will be flagged as copied and you will see a message when you go to that hub.