Found under: Settings > Song Actions > Duplicate
If you want to have two copies of a song, or want to modify a song, but keep the original, you should make a duplicate.
Open the song you want to duplicate and then from the settings menu, select 'Duplicate'.
You will be asked to enter a new name for this duplicate song. Be aware that you should enter a unique name (even add the word 'duplicate' to the end if you are unsure! This will save an additional song to your device with the new filename.
Once you have made a duplicate, you can edit this song as normal and the original is not affected.