DeDup merged Libraries

I'm looking for workflow advice or tips to use iPhoto as the application of choice to manage my photos (20,000+ counting duplicates). I've read though or listened to tutorials on iPhoto and have a fair working knowledge of computers and shared applications.

I switched over to apple a couple of years ago. This switch left windows folders of photos on my work computers, home computers, laptops and phones. There are more than one because as our kids grew up and left home they left many photos on our file servers scattered all over. At work and home, many times our kids would just hand me a thumb drive/cd with photos which I promptly dumped into a folder and imported into iPhoto later on the nearest computer.

Now, I'd like to reclaim wasted space by deleting these scattered folders once I know they have been imported to iPhoto.

I'm using Drop Box as the place for the "default merged library" so I can access the same library or libraries from multiple locations. I will be the only person to use the Drop Box so I'm not concerned about multiple users of the same database.

I've used "Library Manager" to merge libraries from different computers and preserve the meta data, tags, edited versions, etc.

One main issue, is the vast number of duplicates I'm discovering and how to trim these from the main library. I have started using additional libraries to help break apart the huge single mass into smaller more manageable bites but this just helped uncover more duplicates.

Are there any shortcuts to help discover and remove duplicates? Automated tools or scripts? Can I safely remove all the old folders once I see the photo in iPhoto? If I import the same folder twice from two different computers it does not always obey the "do not import duplicates" check box as the clocks may not have been the same when the photos were loaded on the computer originally. Any ideas about how to compensate for these duplicates and save time is appreciated. Recommendations about sensible ways to use multiple libraries would help too.

End goal is to have a single set of iPhoto libraries, without duplicates, stored on Drop Box from which I can export albums/events to family and friends via a web link. I would only like the photo image stored and taking up space without the worry about files, file folders and sub structures.

Thanks,

pblpup