I've also experienced having a hub unpublished because of duplicate content. That was the first time I ever got that warning so I was also confused about it. I replied to the e-mail that I got from HubPages and asked where the duplicate content was.
The duplicate content apparently came from one of my other hubs since I was writing about the many different music albums from the same series so there were some things that were inadvertently duplicated like when I give a short information / overview on the series in question before discussing the music album.
To fix this, what I did was write another hub that contained the information on the series and its characters, etc so whenever I have to mention this on my music album hubs, I just include a link to the information / overview hub.
You can try doing the same for your hubs so you won't have to inadvertently duplicate your own content. Or you can just make sure that you write things completely differently. Hope this helped.