validate duplicates in oracle etl

I am new to oracle and I would like to know how do we validate duplicate values in target system in oracle tables as part of the ETL testing process. (The two tables could be T1 and T2). Please let me know a sample query.

Example: There are multiple rows for a key attribute in newly loaded table T2 but not in T1, they need to extracted and reported(validated).

Thanks, Santosh