iTunes 11, duplicates and my horrible timing

I'm an advanced Apple user but this iTunes question has me stumped. A little back story first. Thanks for taking to time to read this. I have a lot of music. Over the years my collection has become spread out on one MacBookPro, an iMac and several external hard drives. I recently started a plan to get it all consolidated in one place and purge duplicates as well as lower bit rate versions of tracks. My plan was to create a new iTunes library on an external drive and add the music from each location to it, with "Keep iTunes Media folder organized" and "Copy files to iTunes media folder" checked off. My goal was to add each batch from each location to the new library on a different day. Then, once I was confident every single piece of music was accounted for in the new library, I planned to select "Display Exact Duplicates" then sort by date added so I could purge all the dupes that were from the different locations. Makes sense so far?

I spent weeks prepping this process, cleaning up tags here and there and, earlier this week, finally got all the files (over 55,000) into the new library. My next step was going to be a breeze the next day - just do the Display Exact Duplicates purge I mentioned earlier. Here is where the problem is. I made the critical error of upgrading to iTunes 11 the night before and was stunned to discover that Apple has removed all Display Duplicates functions from iTunes 11! So I then figured "Ah...Time Machine to the rescue" and managed to restore iTunes 10.6 back, giving me both versions to choose from now. Holding "Option," I launch 10.6 and, when prompted to "Choose Library," select the iTunes 11 library that had all my consolidated files, only to see a message that it cannot be opened because it was created with a newer version of iTunes. So I am stumped.

How can I open an iTunes 11 library file with 10.6 There has got to be some way to do this. The important factor here is retaining the library with the different "Date Added" tags so I can delete duplicates correctly. I also made the stupid mistake of turning off Time Machine earlier so unfortunately there is no backup of the correct 10.6 library file. I could just build another new library and repeat the process over a few days but I already deleted one batch from one source so that will only work to a certain extent and still leaves me with thousands of duplicates (but admittedly not as many as before). Tune Up is useless as well. I hope this was a clear explanation and appreciate any ideas. Sorry if it was long.