Hi Paul,
the time consuming bit will be identifying the duplicates as you will not necessarily know what the duplicates are.
The delete is easy once you have identified a track to be deleted.
Using the Web UI just click on the threes dots next to the track name and the Delete dialog will be displayed
In terms of finding the duplicates this is more tricky.
One way is to use a backup of the your B2 contents on a USB drive.
You can then take a look at the contents of the USB drive on a computer using some software such as MP3Tag.
The B2 when it rips a CD does populate some metadata tags including track title.
Once you have opened the USB drive contents in MP3Tag then you can sort them on the "title tag" in order to group all the tracks with the same name together.
Identify your duplicates and make a choice of which ones you want to delete on the B2 using the Web UI
After you have done all your deletions its probably best to make a fresh backup.
If you need more detail on how to use MP3tag to help with us let us know.
John