This is a similar message, but slightly different meanings behind them.
The single path to disk shelf alert is exactly as described: only a single path to the disk shelf is identified by the controller.
The disk redundancy message can occur for this as well, but can also stand alone meaning one or more disk(s) are not seen on the partner path.
e.g.
disks 0-23 are seen on path A, however path B only see's disks 2-23.