Look at dupeGuru, but it is not free.
You might find this page informative:
You could download OnmiDiskSweeper (free) from the Internet and open it. It will show all of your files and the respective sizes. Delete what you do not need.
Another approach may be to install a larger HDD.
It's unlikely you have ANY duplicate files, unless you do some non-graphical stuff, like do unix stuff, or generate vast quantities of scientific data, most likely again via unix tools. Most apps on the Mac platform do not generate duplicate files. So I doubt your hard drive is filled BECAUSE of duplicate files.
Apple has built into iTunes play count for each song, but unfortunately they again failed to be user-friendly and consistant and there is no such tool in Finder for files. It would have been perfectly placed as part of the Smart Folders feature, right? Just another example of how Apple makes crippleware!
The only solution I have found is to use two unix tools, ls and diff. You'll have to open terminal for that. To get documentation on how they work, from termianl, type man ls or man diff. There's a lot to read, and unless you are a seasoned unix user, you probably won't understand much of it. Here is how it works simply
ls -l folder1 > text1.txt
ls -l folder2 > text2.txt
diff text1.txt text2.txt
The first two lines that start with ls display the folder contents and dump it into a text file. The diff line compares the two text files and shows you any differences. Simple right? Kinda.. it's complicated...