Iphoto libraries

Both Libraries? You mean libraries of both kinds?

Open each of the libraries in turn, this will get them all to v8 Photos Libraries. You can then import from the other Libraries to your main one, as v8 allows importing from one library to another, plus it also will seek out duplicates for you as well.

So, how are duplicates addressed if I do not want to combine libraries? (I have 4 different version 8 libraries that I do not want to merge - the system library and one other are on my MacBook, while 2 other libraries are on an external drive connected to my Mac) Will photos find duplicates in libraries that are not the system library? Or does the particular library have to be open for photos to crawl through it? Or does photos crawl through all libraries that it knows of?

iPhoto Libraries? Or Photos Libraries. I ask as you say iPhoto in your query but are posting in the Photos forum. Also can you clarify which version of the app, and what OS you are on.

The library has to be open. A closed library is essentially inert. Put another way, Photos essentially only knows about the open library. It has no awareness of the others.