Erroneous CMS 200's - tracking down the source

Like any webmaster, Google WMT is my friend and I check it daily, but recently I have been plagued with a bunch of URLs on my site that Google has found, and while technically they do not exist, my CMS serves them up anyway.#

E.g.

Correct URL:

http://www.domain.com/cat1/cat2/id1/id2

but

http://www.domain.com/cat1/cat2/id1/

also works to generate the same content.

WMT serves these up as pages with duplicate title tags, but I cannot find out the source no matter what I try. I am trying to work out a way to stop the CMS 200’ing those pages, but in parallel, I would love to plug the hole at source.

Any ideas?

Tried creative search of Google. They are not coming from with my site, I even checked the RSS feeds.