Unidentified data in My Book

Western Digital Issues
My Book P/N WDBACW0030HBK-D

About 9 months ago I had to replace the Hard Drive in my PC. The data from the old hard drive was transferred directly to the new hard drive by the service provider. When the WD back up was reconnected to the PC it did a full backup. However, it looks as if it did not recognize that the data already on the back up hard drive was mostly the same data it had just backed up. Instead it would appear that the data already on the backup was merely labelled “Other Backups” (880.8 GB) and “Additional Files” (626.8 GB)
I had a lengthy email exchange and conversations with WD technical support but they were unable to tell me how integrate the data contained in the two other groups with current data.
The problem is that these other datasets contain information not in the current hard drive so deleting them to free up storage space seems to defeat the whole purpose of a backup.
I had a second, 2 TB hard drive installed on my PC so I have a place with sufficient space to which files can be retrieved
One of the things which puzzles me is that when you change C: drive to the D: drive the files in My Book appear differently than when C: is the backup source. When D: is the target it shows 4 file names, each of which have a different date
I tried to run a duplicate file search on the My Book drive but it wouldn’t work
What I’d like to do is is to retrieve each of 4 discrete volumes to the D: drive and see if it’s possible to discover files which are duplicate to those in the current active C: drive so that the duplicates can be deleted.
I wonder too if the replacement drive had been restored from My Book this problem would not have arisen.