How to use Duplicate Finder

When the duplicate list (very long) of files is displayed does this mean I no longer need them and could delete them without damaging my system?

That's part of CCleaner that could be dangerous to your PC if you don't know what you're doing.

I'd recommend reading here which is another recent topic about it:

http://forum.piriform.com/index.php?showtopic=43635

Dup File Finder is a bit like the registry cleaner, you really don't ever need to use it.

But if you do, prepare for disaster recovery.