Values need to copy from Col A to Col B in Excel without data loss

In my Excel, I have 2 columns For Ex: Col A & Col B. In Col A I have 100 Records but in Col B having only 50 records that is scattered not in sequence. I have to copy and paste the same information for missing in Col B from Col A. When I put look up and tried to copy and paste the values it’s not coping the data correctly. Can anyone please let me know how I can do this?