I don't use Windows, but I've used fdupes on Linux. The WP page says that there's some Windows build, but I don't see Windows binaries on their release page.
When I hit alternativeto.com, it listed Czkawka as a graphical alternative to fdupes, and it looks like that does run on Windows. I have not used it, though.
EDIT: Ah, here's the Windows-compatible fdupes fork, jdupes. I do see Windows binaries on their releases page.