Delete correct duplicate files

Hope someone can help, am unsure how to handle the below.

Scenario (the names are meant as an illustration, each folder contains quite a number of subfolder/files)
X:\Documents\Excel
Y:\Backup\Documents\Excel

I run find duplicates, MD5

Opus finds a lot of duplicates.
The files on X:\Documents\Excel should be kept at all times
All duplicates on Y should be deleted.

However, if Opus find duplicates on Y only (Backup), i.e. they donot exist on the source folder (X), then
just one on Y (Backup) should be deleted as obviously a backup file should be restored to source.

Q1: How to mark all duplicate files on Y in 1 go (i.e. no one-by-one tagging)
Q2: Is it then safe that just one is deleted, not all files that are identical in the same folder

The duplicate finder's delete mode will automatically select all but one of each duplicate group. It keeps the first file according to your sort order.

Thanks. This helps me al lot already. It would be nice though if I could have some influence on the tagging.

e.g.
if in source folder a specific is available -say- 3 times (in different subfolders, for different purposes, e.g. an explaining text file, whatever)
and in a destination (backup) folder also say 3-4 times
(I do have instances where a file shows up maybe 5-7 more times all together )

at that point Opus tags all, but one in the source folder

The "Find box" is like this:
x:\source
y:\backup

Now, if -as a first go- it would be possible to rightclick select the y:\backup and then (new option) something like 'Select all in this folder and subfolders'
basically I would then be left with duplicates in source only.

In my case I would like to deal with those duplicates (source) in a separate find duplicate session or maybe leave them as they are.

Right now I have to scroll down all the hundreds of duplicates (to verify that I am not deleting from source).