How to scan for duplicate files / images in one direction only?
It looks like you're into photography and you haven't been keen for best practices for managing files. You have two options now.

1) You'd have to create (bash OR python script) which would look for MD5 checksum of the files and would delete only it finds duplicate with later date. Keep in mind, one careless step can delete the files you didn't want to delete. 

2) (Suggested) There are several free and paid utilities that should be used to cleanup the duplicates. If it provides duplicates with date comparison. As you already mentioned (Gemini). I tested it few months ago and found it very useful.

There are several proven practices to manage the photo library. This is a good time to adapt one.

As you requested, here is a script which find duplicates in one folder.

#!/usr/bin/env python
# Syntax: duplicates.py DIRECTORY
import os, sys
top = sys.argv[1]
d = {}

for root, dirs, files in os.walk(top, topdown=False):
  for name in files:
    fn = os.path.join(root, name)
    basename, extension = os.path.splitext(name)

    basename = basename.lower() # ignore case

    if basename in d:
        print(d[basename])
        print(fn)
    else:
        d[basename] = fn


Save this file as duplicates.py and give it rights and then execute it on the folder.

./duplicates.py Images


It requires you to have an understanding for the osx terminal.