when the field is a duplicate

There are duplicates of a specific field in a sql database. If the selected row has a duplicate in the code field then I want to select one of them as 'a' as field2 and one of them as 'b' as field2.
something like: case when the code field is a duplicate in the database, then select one of the duplicates as 'a' and the other as 'b' as field2 . there is no primary key in the table.
What would be the best way to accomplish this?
Thank you.

How is your table structured? From reading your post, it appears you want the duplicate records to get sorted in two different buckets. What happens if there's a third record with the same column values?

The first value should be 'a' and all the rest of duplicate values(second, third...) should be 'b'.
Thank you.