D6 custom block migration overwrites existing D8 custom blocks?

It appears that the D6 custom block migration overwrites existing D8 custom blocks. Here's how to test:

Setup

Drupal 6 site

1. Start with a fresh D6 site.
2. Add two custom blocks, place each in any region (see http://note.io/1f6DEYs).

Drupal 8 site

1. Start with a fresh D8 site.
2. Add two custom blocks, place each in any region (see http://note.io/1ftnd31).

Migration

Perform a migration using the manifest from https://drupal.org/node/2221805.

Results

See http://note.io/1f6Eu7O

1. Both D8 custom blocks are still placed in regions.
2. Neither D8 custom block is available in the "Custom" section of "Place blocks".
3. Both D8 custom blocks are available in the "Custom section of "Place blocks".
4. Only one of the D6 custom blocks was placed in a region.

Thanks,
-mike