How do you determine if a transaction is unique? I originally thought one could determine whether a transaction is unique or not by its id. However, if you perform sendmany, then each of these transactions will share the same id, is that right? In this case, how would you go about determining if a transaction is unique?
Each transaction can have multiple inputs and multiple outputs.  sendmany is used when you want to send to many accounts.  It's all still one transaction.