The files with copyright headers are not skipped.

Hey all,

I am running the command below.

`addlicense -f license.txt .`

This should ignore the files which has the header defined in license.txt. However, it adds a new header on top of files where license header exists already.

Reproduce the problem with `addlicense -f license.txt .` and inspect the existing-header detection path. Verify how headers from `license.txt` are matched against scanned files; done means files already containing that header are skipped rather than receiving a duplicate header.

Written by the indexing model from the issue text.