PAR files are a form of Forward Error Correction (FEC) (implemented with Reed Solomon). They can be used to repair some percentage of damaged blocks in a file or set of files. The percentage is determined ahead of time when you generate the FEC (PAR files) and imposes the same amount of storage overhead (i.e., 15% redundancy costs 15% additional disk space with Reed-Solomon).

https://en.wikipedia.org/wiki/Error_correction_code#Forward_...

https://en.wikipedia.org/wiki/Reed%E2%80%93Solomon_error_cor...

CDs already use Reed Solomon, though. But yes, you can double it up.

33% parity, apparently.

https://en.m.wikipedia.org/wiki/Cross-interleaved_Reed–Solom...