Whether it's your own writing copied, or taken from somewhere else, HubPages does not like un-original written content. The duplicate Hub warning is an automated flag that appears when too much of your Hub can already be found on the web. This can also happen if you quote a lot in the course of your content.
If you have the flag due to quoting someone else, ask admin to take a look and see if they'll certify the Hub original, which will get rid of the flag.
If the writing was yours that you used again, go back and rewrite it so that it's fresh and original. The flag will clear when you republished the Hub and the system gets the chance to rescan the content.
If you used someone else's work, that's plagiarism, so first go read up on that, then go back and rewrite your Hub.