Fix spurious reporting of baker double signing

[td202]

Purpose

Fixes #949

Fix an issue where duplicate invalid blocks are spuriously reported as a case of double signing.

Changes

• When double signing is detected, check that the block is actually distinct from the previous signed block.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.