Lightroom doesn’t have a strong built-in duplicate finder for this job, so the usual approach is to use a plugin or an external file-level method.

From the community answers, two Lightroom plugins were suggested:

• Teekesselchen
• Duplicate Finder / Dupes-related Lightroom plugins

These can help identify duplicate photos inside your catalog, which is usually the safest place to start if the images are already imported into Lightroom.

If you want to find exact file duplicates across folders outside Lightroom, a checksum method works well. One answer suggested using md5sum from the command line on Linux/Cygwin to compare file hashes. That finds exact binary duplicates even when they’re stored in different folders.

Best practice: identify duplicates first, then remove them carefully from within Lightroom if they’re cataloged there, so you don’t break catalog references. Also double-check before deleting, because edited versions or files with the same image content may not always be true duplicates at the file level.