The most simple solution would of course be to remove the title completely and instead use the caption to say whatever it is you want said which will appear as a semi-transparent overlay (whose color, position, animation you should be able to control .. see image from my previous post and experiment)
If you insist on having a title there it can be pulled out of the database along with the custom url. Do you want to first see if you like the caption option or title with correct url?
You can either hide the redundant portfolio item page entirely , so that the only thing it containts is just the image and since its listed nowhere on the site, the google spider won't be able to index it (you can also add no-index meta tag to it).
It's a myth that duplicate content hurts a website. It doesn't. It just gets ignored by google with no penalty. Every day hundeds of AP and Reuters news stories are carried by thousands of websites without rephrasing: yahoo, cnn, fox, abc... etc.. If there was a penalty every news site would be blacklisted,
Google invented the canonical meta tag which is used on all similar pages of the site to point to the 'main' page to let google know that http://www.hello.com/about/ is the same as http://hello.com/about/ ... you can add that meta tag to individual pages using yoast seo plugin (or some other).
However you can use the portfolio item page to your advantage. Add unique content to the portfolio item page and use it as a secondary information source that provides additional info about the contents of the post. example : ingredients list and calorie information for the post about a specific breakfast, or review page about it, or...