prevent duplicate user from being added in Code#assign_user

After receiving a match code at LSRC, I noticed that if I entered in my code a second time, the page would still tell me that my pair hadn't signed in yet.  However, I received an email saying that I was matched to myself.  

I think this is because my github user was being passed to the mail observer in Code#assign_user twice.  The quickest fix I could think of was calling users.uniq!, but would love any feedback if there's a better way.  Thanks!