Debian Policy 4.13 states that Debian packages should not use convenience copies.
The list of packages embedding code from other projects is maintained in the secure-testing svn repository:
This list also contains information about code forks so that the security team can check if all forks contain the same vulnerabilities. Send suggestions or additions to the debian-security-tracker mailing list.
The Debian duplication detector detects duplicate files in binary packages and may be useful for detecting verbatim duplication of interpreted code and data.
If you have a particular piece of code with some interesting aspect (security issue etc) you can likely find other copies using the Debian code search site or external code search engines such as Ohloh code, searchcode and GitHub.
Various Debian folks keep track of embedded code copies they found via usertags: