use of multiple libraries versus groups in a library

I am setting up EndNote libraries for a group of scientists who work on different (but sometimes overlapping) topics on the same species (usually, though sometimes on other marine mammals).  

The collection of references on the species numbers almost 2,000.  If I add all the “topics” references, the whole library could be 8,000 to 10,000, which seems unwieldy, even pulling out references into groups.  Any single user would use the references for the species and his/her topic, but not the thousands of others related to other topics.

However, if I use separate libraries, there will always be overlap with references showing up in more than one library. Any action (like attaching a pdf) would have to be done to all the copies (which means you’d have to find them) and I expect that chaos would eventually reign.  

I’d appreciate any suggestions from experienced users on which approach is more fraught with peril!  Thanks!

I have always recommended one library so I wasn’t duplicating refs and PDFs.  You can use more than one library to format a paper.  Thus if you have multiple libraries with a clear delineation of topics, you could open a second (or third) library when someone needs a ref from another “topic”.  However, I find using smart groups in combination with added occasional keywords, allows the group functions to make things manageable. 

Yes, best to use one library for all references, and groups to create subsets.

If you use multiple libraries, in addition to needless duplication, there’s the peril of EndNote not recognising an author as being the same if the author is entered into separate libraries – a hassle for example if using the standard APA setting of adding initials to distinguish different authors with the same surname; plus, different references by same author + same year will not, I think, be handled correctly with alphas (2001a, 2001b, etc, for example).