find duplicates in column and appending to internal table

I have internal table with 5 fields, let them name F1, F2, F3.. and so on.

This table has about 300 rows. Now, based on F1, I want to search all rows for duplicates. Then I want to delete all rows which have no duplicate and reduce all founded duplicates to only one record.

For example: If there is a record 3 times, I want to write it only 1 time in a new internal table.

I've tried to do it with LOOP AT but I could not figure out a good solution.

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