Try disabling all plugins except for Post Duplicator, and also switch your theme to a default TwentyTwelve, or Twenty Eleven and test the Post Duplicator. Then duplicate a page and see if you still have the same issue (I'm guessing you won't).
Then, re-enable your theme and check again. If it stops working then your theme has something built into it that stores that page title and you'll need to figure out how to manually change that.
If my plugin still works after re-enabling your theme, then re-active each plugin one by one, testing the duplicator between each one to find out which plugin is creating/storing the page titles. Then you'll know where to start looking to find how to manually change them.