//I'm assuming you use MySQL, it's probably different for other systems
//Okay first, the error code for duplicate entry is 1062. And here's how you retrieve the error code from the exception:
try{
User::add_user($user_details);
}
catch (IlluminateDatabaseQueryException $e){
$errorCode = $e->errorInfo[1];
if($errorCode == 1062){
// houston, we have a duplicate entry problem
}
}