when my phone backs up info, does it back up duplicate items

If you are referring to the backup through iCloud or iTunes then the current state of your phone is backed up in a single backup file. If you were to make another backup of your phone then you would have two backup files which would each contain the data, pictures, files, passwords (if the backup is encrypted) which were on your phone at the time of the backup. It is possible that the backup files could contain duplicated of the same items depending on if you had the same file/item on your phone during both backups.

Hope that helps,

-Yama