Unique reference in different libraries

I wish my references are uniquely identified meanwhile I can access them from different libraries to update them. You know one reference might exist in several topics.

You mean you want Endnote to notice that you have the same record in another library and link them in some way?  

Or that you want to insert a different version from two different libraries?

I use only one library and recommend that to all my trainees -  and group the topics by keywords using smart groups, then they can be in multiple groups, but there is only one copy in one library.  Why whould you want to have duplicate copies of PDFs and other materials, and the need to update each record in multiple libraries, coupled with remembering which library(libraries) the record is in?