Develop a table in Google Sheets

There is already an existing table in Google. This is a referral system. But it is not working

Column A - Client's First and Last Name

Column B - Client's Phone Number

Column C -  Referred Person's Full Name 

Column D - Referred Person's Phone Number

New clients are imported into columns A and B from our recording system via a csv file

Data is manually entered into columns C and D, and when fields C and D are filled, the client should automatically be added to columns A and B

Task:

Make it so that when importing the database, clients are not duplicated in columns A and B if they have already been added via C and D. There should be an automatic duplicate check.

The second task is - when fields in columns C and D are filled, the client is added to columns A and B