[WIP] add initial test to block indexing identical files in the SBT.

[ctb]

Addresses behavior where same signature file can be inserted multiple times into an SBT.

Fixes #145, or will eventually :)

The main question here is, what is the appropriate behavior?

• we could block and/or complain at the command line.
• and/or we could raise an exception at the API level.

I think I'm in favor of both, since this is clearly invalid behavior. I'm not sure how to implement the latter, will look into it.

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[luizirber]

Note: a duplicated MD5 doesn't mean it's the same signature (empty sigs will have same MD5, but different name/filename. Ref: #994