Limit number of entries in a MySQL DB

I am trying to limit entries into a MySQL database to avoid duplicate entries. Users will input values for a $week and $team. Teams can have multiple weeks, but there can not be multiple weeks per team. How can I check the MySQL DB prior to inputting values and if there is already values matching the $week and $team criteria exit and redirect to an update page? Hope this makes sense. I believe this should be fairly simple, but am not sure where to start. Any help would be appreciated. Thanks, Scott