It's not possible to determine if a file in a torrent also exist in another torrent from the data available in the .torrent files. There simply is no deduplication support in the standard BitTorrent protocol.
When a torrent is created, all files in it is concatenated together and then chunked up in pieces.
example:
Files |---#1|#2|---#3|-----------------------#4|#5|------#6|-----#7|-------#8|
Pieces |--0|--1|--2|--3|--4|--5|--6|--7|--8|--9|-10|-11|-12|-13|-14|-15|-16|17|
- All pieces has the same length except the last one.
- A file has one or more pieces.
- A piece may contain (parts from) more than one file.
- It's very rare that internal file and piece boundaries align (except if padding files is used).
- A file in a multi-file torrent almost always has a piece shared with another file.
The problem is that it's not possible to create a file hash from the piece hashes.
(@jdwolf Even if file #1 and file #3 is identical, all their piece hashes will be different.)
There is a few torrent creator programs that optionaly can add file hashes, but afaik it's not used anywhere.