Law students at the University of NSW are furious about having to sit their exam again after the faculty accidentally re-used a paper from the first semester that had since been published online.

Some students are considering a boycott of the second exam, according to an open letter that was gathering signatures on Monday night.

Students at the University of NSW (UNSW) are angry at having to re-sit their law exams because of an administrative bungle. Credit:Dean Lewins/AAP

The fourth-year business associations examination, held on November 3, was already controversial because it was so difficult. Many students immediately worried they had failed and would have to sit the compulsory subject again.

But on the afternoon after the exam, the law faculty told students that an administrative error had led to the semester one paper being re-used. That exam had been available for students to download from the library.