Hello!
Within the Global Address Book Parameters you have a setting:

This will give you a information message when you are creating a new worker. This will give you the option to validate if the employee you are hiring, has the same first and last name of someone in the Global Address Book.
Like this (using personnel action new hire):

If you click on the blue '1 match found', you have the option to validate on maintained detail, and identity if this is the same match..

In order to clean up the Global Address Book:
- You can delete them (make sure no relations are active);
- check for duplicates (but this works on a 1 by 1 basis.. so not like excel where all duplicates are highlighted);

Please let me know if this answers your questions, and mark this post as helpfull is so ;)
If not > reach out to me.
With regards,
Erik