Edit: Post updated due to obvious user error 
Looking for a program to find dublicate files? Then fdupe saves the day!
root@bais:/home/samba/raid0# apt-cache search fdupe
fdupes – identifies duplicate files within given directories
By the way, how come the apt search gives to random results?
torhenning@bais:~$ sudo apt-cache search duplicate
….
mirror – keeps FTP archives up-to-date
vlc – multimedia player and streamer
(and a lot of other irrelevant crap)
Can`t exactly say that having to grep in a search results is a good “feature”..
Part of the reason why might be because it’s spelled ‘dupli’, not ‘dubli’.
Gosh, very good point!
Updated the post with some new rating instead
Might be because it searches descriptions by default:
$ apt-cache show mirror | grep duplicate
Mirror uses the FTP protocol to locally duplicate remote host files and
And btw. since this is old quality software it has ofc. a documented parameter to only search names:
–names-only, -n
Only search on the package names, not the long descriptions.
Configuration Item: APT::Cache::NamesOnly.
I will leave you with a quick RTFM