Freedup walks through the file trees (directories) you specify.
When it finds two identical files on the same device, it hard links them together.
In this case two or more files still exist in their respective directories,
but only one copy of the data is stored on disk; both directory entries point to the same data blocks.

If both files reside on different devices, then they are symlinked together.
Please see the Syntax page for instructions, how to modify the bahaviour of freedup.