It seems to me that your database design (or the name of the 'id' column) is broken if you have non-unique ID's.

FALSE:

CREATE TABLE students
  id PRIMARY KEY SERIAL
  firstname TEXT
  lastname TEXT
;
CREATE TABLE classes
  id PRIMARY KEY SERIAL
  name text
;
CREATE TABLE student_classes
  studentid  INT
  classid INT
;
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and now:

SELECT  * FROM student_classes;
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Will you get several instances of each id? Yes? Is the database design un-normalized? No. Conclusion. There are reasonable queries leading to multiple instances of certain fields by which one might want to collate results. In this case, I might want a hash keyed by student id and an array ref of all results with that student id. Call it "collation".