I guess your going to have duplicates, no matter what.
Unless every single book has the correct Author and Title and there the same throughout you library's (ie Marion Zimmer Bradley / M. Z. Bradley / Marion Z. Bradley etc)
What you can do is merge them, and then use the duplicate plugin (see plugin thread) which will remove 90% or more of them esp using the fuzzy logic.
I imagine there will still be some, mislabeled or with alternate titles that will be duplicated
With 23 Thousand books I would be tempted to leave some in an unused library, ie books that your not really going to read anytime soon - and considering that it would take you
221Years at 2 books a week, there will def be a few you wont read - EVER