SmartDupe

Just as the developer says, this is a shell script. No GUI. Nothing wrong with that. But, if you aren't comfortable using a shell script, than look at some of the similar software. However, this is free!! And since it is a shell script, you can edit the code if you want to and know how.

Even glancing at the commands it's hard to determine exactly what the criteria is for determining if a file is a duplicate. It seems unlikely to succeed identifying exactly the same content for example. Which is what a duplicate is. Even if this succeeds, why would you need hard links? What is supposed to be designated the original?
There's no example config file supplied. I don't see much value in this except to learn some file handling commands from. I'm sure it has its uses, like when managing code.

Just as the developer says, this is a shell script. No GUI. Nothing wrong with that. But, if you aren't comfortable using a shell script, than look at some of the similar software. However, this is free!! And since it is a shell script, you can edit the code if you want to and know how.