Here’s a script from GPT4:

#!/bin/bash

# Create a temporary file for storing file checksums
tempfile=$(mktemp)

# Generate MD5 checksums for all files in the current directory and its sub-directories
find . -type f -exec md5sum '{}' \; | sort > $tempfile

# Detect and delete duplicates
awk 'BEGIN {
    lasthash = "";
    lastfile = "";
}
{
    if ($1 == lasthash) {
        print "Deleting duplicate file: " $2;
        system("rm -f \""$2"\"");
    } else {
        lasthash = $1;
        lastfile = $2;
    }
}' $tempfile

# Clean up
rm -f $tempfile

This script can be run with Termux from the root of your internal storage. Usually /sdcard or /storage/emulated/0. Do not confuse this with running from root if you are rooted.

Before using a script that interacts with your files you should backup anything that is important just in case.

Furthermore, if you comment out:

system("rm -f \""$2"\"");

By adding a # in front of it like this:

# system("rm -f \""$2"\"");

You can run the script and see what files the script would delete without actually deleting them. I would recommend doing this as I have not tested this script.

fdupes in termux

FX File Explorer has that functionality (among other cleanup tools) and is generally a great file manager. It’s free with limited functionality after a trial period, or a one-off fee of about three bucks for the Plus version.

MiX file explorer for android does this. It’s free. Not sure if it’s open source though, does anyone know?

It does use open source libraries.

Latest version is 6.64.3 beta.

https://forum.xda-developers.com/t/mixplorer-q-a-and-faq-user-manual.3308582/

“MiXplorer (Mix of Explorers) is a fast, smooth, beautiful, reliable and fully-featured file manager with a simple and intuitive user interface.” created by XDA member and Recognized Developer @HootanParsa

If you’d find one I’d be interested in hearing what it is. As an alternative, you might be able to use a Linux app while connected through MTP.

My Android 12 Samsung phone can show me all duplicate files right in the android storage settings

I like Czkawka for detecting and handling duplicate files, similar files, empty directories, and more.

The Google Files app will find duplicates and other things.