The solution viewer should have a way of distinguishing between different copies of the same piece. Possible solutions:
Slightly vary piece color (BurrTools does this, but it's not very distinct)
Highlight a whole piece on hover
Toggling piece visibility / outline / solid, as in #21 . BurrTools does this, and even has the wireframe of the piece itself, not just the wireframe of the individual voxels that make up the piece.
The solution viewer should have a way of distinguishing between different copies of the same piece. Possible solutions: