Not an easy task and can sometimes cause unintended problems using third party Apps. But outside of third party Apps, you could start by using OS X Spotlight.
For third party App recommendations, do a search in this Forum or Google. Be careful.
Great advice Joe, it also depends a lot on Cesar's habits.
If he has rightfully imported music into iTunes and photos into iPhoto, then there should not be very many if any duplicates. Also it is just as important to not confuse ones iTunes and iPhoto Libraries as duplicates and mistakenly delete them.
Then again if one has a bad habit of dropping stuff on the desktop or just anywhere, then is is going to be very difficult to clean up.