Tick and
Set (to your choice)
Preferences: Adding Books: Automerge (I use the 'Create new record for each duplicate FORMAT'. This really should not come into play as you describe the merge sourse)
Then do your
Add
Then use the
Find Duplicates PI, because there are always outliers
in the results (order sensitive):
1) Select the Target (keeper/best MD )
2) Select the books to merge into #1
3) Delete any
Unwanted of the duplicates