Using commercial software
Try "Find Duplicate Files", it's free for 7 days.
Araxis Find Duplicate Files helps you to find, view and manage files that have duplicate content, regardless of name. It can be especially useful if you have large Pictures, Downloads or Documents folders.
There's a handy tool called
fdupes which finds duplicate files and allows you to delete them.
Install Homebrew using the guides on the homepage. It should work if you type:
ruby -e "$(curl -fsSL https://gist.github.com/raw/323731/install_homebrew.rb)"
Homebrew is a package manager that can install some of the most important Unix tools, including
fdupes. After you have installed Homebrew, just type:
brew install fdupes
And after that, you will be able to scan your directory with:
fdupes -rd your-folder
This will prompt you whenever duplicates are found and ask you to delete or preserve them.