CSV Text Record Manipulation

[meka biz]

Hi I need to deal with customer lists in CSV files... doing tasks like:

# Find matched keys in a column. Extract & add their values(from the row) from smaller csv to the main csv file.
If there are multiple matched records for the same key, make new rows & copy those records
to the rows below the main record.

# If there is no matched key, add the new records to the main csv.

# Find & group duplicated keys with their records into rows next to each other for manual inspection.
What should I do to achieve this?
Please help & Thank you.