When you install Data services Normalization plugin and run through with the guided setup, it downloads the Normalized company names from servicenow repository. Once you complete all the related steps from the guided setup, it marks the Normalization flag to either true/false based on the common name.
Example
Normalized company name = Microsoft Corporation
Core Company has "Microsoft", "Microsoft Corporation"
It marks Microsoft Corporation as Normalized and update all the references where ever using the "Microsoft" as "Microsoft Corporation"
To your other query, how to avoid duplicate, once you install this plugin, it automatically controls all unwanted Company / Manufacturer names not to show in any other references. You may see the unwanted companies only in core_company table, but it is of no use and can be deleted over a period of time.
Thanks,
Narsing
