class backup {
  user {"dbbackup":
    ensure => present,
    home   => "/home/dbbackup",
  }
}

class backup::mysql inherits backup {
  User["dbbackup"] {
    groups +> "mysql"
  }
}

class backup::pgsql inherits backup {
  User["dbbackup"] {
    groups +> "pgsql"
  }
}

include backup::mysql
include backup::pgsql

#puppet test.pp
Parameter 'groups' is already set on User[dbbackup] by backup::mysql at /test.pp:11; cannot redefine at /test.pp:17 on node testmonkey