Merge ‘Box Sync (backup)’ into ‘Box Sync’

I replaced a failing disk, restored files that were on the original disk to the new disk, some of which were synchronized with Box.com.  The root of the Box files was called `Box Sync`.

The next time Box’s synchronization ran, it decided to rename the root  `Box Sync` →  `Box Sync (backup)` and then proceeded to download all the files in the cloud to a new folder, `Box Sync`, which left me with a lot of duplicates in two deeply nested folders, namely:

1. `Box Sync` (4.83 GB)
2. `Box Sync (backup)` (4.82 GB)

Can anyone suggest a good way of merging these two folders together?