we are trying to write a code to fetch unique values from a field for example division and populate those fields in the custom table.
Then your design is wrong. If you want to relate them, use a reference field then determine some way of populating the reference.
Using a business rule to mirror data between two fields conflicts with ServiceNow's "Single System Of Record" concept. It doesn't mean it can't be used (or done), just that it's not considered good practice because denormalised tables carry an additional maintenance overhead.
