It depends of the species, but polyploidy is common in plants because plants often can work as colonial organisms that can clone itselves.

Is the same problem as to restore a file that is partially distroyed in four or five points. You can still recover the original file if you have five copies, each one with the damage in a different area. Polyploid organisms have several identical security copies of each chromosome. If something in a chromosome is damaged, they can quickly fix the damaged areas using the safety copy saved in his 'twin' chromosome.