I use Auslogics Duplicate finder which works great even for files with different file names/extensions or dates. However, unless your .doc and .docx file are bit for bit identical and just happen to have different extensions, I don't think you'll find a duplicate finder that can identify them for you.
There are plenty of music management software for identifying similar audio files based on metadata (e.g. if you have a track in multiple formats: flac, mp3, wma, m4a, etc). However, if you save the same document in say, Microsoft Word 97~2003 format (doc) and Microsoft Word 2007 format (docx) which are two completely different formats, you're probably on your own.