How Do I Find Duplicate Photos?
Compare the file contents, not the names. Google Takeout in particular hands you the same photo under several names.
Names lie. The same photo can arrive as IMG_1234.jpg, IMG_1234(1).jpg and IMG_1234-edited.jpg. A tool that compares file contents catches all three; one that compares names catches none.
Takeout exports are full of these, because a photo in three albums is exported three times, and edited versions come alongside the originals.
Takeout JSON Metadata Fixer recognises repeats as it organizes, so merging a second export into the same library does not double it. It runs on your own computer and is free for your first 500 photos.