Weekend Mystery: Primary Key Indexes

A few months ago I read this article from Arup Nanda, but it wasn't until re-reading it tonight that the lesson he was teaching us really sunk in for me.  If a table has a primary key, you'd naturally think that duplicate values can't exist as the primary key right?  WRONG!  Arup shows us exactly why, as well as explaining why this is actually desired in some situations which taught me even more.  Explore the mystery this weekend!