When you hash an already hashed file, the file reference is not replaced in the dest

When you hash an already hashed file, the file reference is not replaced in the dest.
For eg. When I hash a file called 'style.css', it renames it to say 'style.h4543.css'. The reference in the dest file index.html will also get changed to the new name. But if I hash the file 'style.css' again after making some changes, the new hashed file name changes to 'style.foo123.css', but the reference in 'index.html' remains the same. 

I fixed this issue by saving a mapping of the the original file names and their hashes in a file called filecache.json in json format and then using those references while hashing.