The Commodore PET/VIC-20/C64 tape routines write out the leader for each file twice, and then write the data twice. While it would seem that this should allow data to be loaded more reliably, experimenting with different kinds of corruption suggests that any problems on the tape are prone to render it unreadable, even if there is a complete undamaged header and a complete undamaged copy of the data.
The Commodore tape format certainly provides enough redundant information that a suitably-designed loader would be able to recover from any dropout which is shorter than the gaps between the two copies of a file header, or between the two copies of a file. As such, it would appear to be an example of a cassette format that uses a crude form of error-correcting code (as alluded to in the other question), but the load routines found in the VIC-20 and C64 don't seem live up to that promise.
Are there cases where the redundant information stored on the tape would allow successful loading of data that would otherwise be lost? Or is the intention simply to provide extra assurance that if both copies retrieve identically the data is almost certainly correct? Or perhaps tape format was designed for the PET, which might have had enough ROM space available for a more sophisticated tape loader?