The question kind of says it all. Imagine that for an assignment you receive two or more similar submissions (e.g. programs). What will you do? Annul all the works? What if you know for certain who was the original author?Probably this is already in your schools code of conduct. In my school it is not and I sometimes dont really now what to do.  
When a student submits a work that is a copy from a colleague, should you also punish the student that made the work?
plagiarism