Library management is a challenge. Duplicates occur usually when archive libraries are imported into an existing library - or when the project references duplicate libraries when opening the file - study the library manager to see the folders it is referencing - probably some extraneous folders.....
In locating doubles: frequently "doubles" occur where entire folders are duplicated - easy to remove entire folders than individual objects.
For moving between separate, unconnected machines, you need to save the originating project as an archive [.pla]. When opening the project on the second machine, "read from archive" and when it is time to go back, save it all as a new archive.
Other options if these machines are networked is to have them use a common library. But it can still have duplicate items if managed poorly.
Dwight Atkinson