RB: "From the tabular point of view, does it make sense why we can't have duplicate rows in a relation?"
John Sullivan: "As with everything else in life, it depends what you are trying to do (and exactly what you mean when you talk about a DBMS table v. a formal relation). From an operational (transactional) database point of view, for obvious reasons, you don't want duplicate rows (enforce a natural key). But if you're analysing data from various legacy sources (e.g. spreadsheets) it might be useful. Then again, you might introduce a surrogate key to give you more control over what's going on - again, depends on what you are trying to do." --LinkedIn.com
One of my readers once wondered why "database professionals understand uniqueness via keys, but don't seem to understand why duplicate rows should be prohibited and the consequences of breaking relational closure":