How rare is it? Is it possible to happend while I am not playing?
I use two accounts for duping. Should i use two USB's to save them? Or is one safe enough? Did PD do anything to solve this?
File Corruption doesn't have any likelihood of it happening. It basically happens when data transmissions are interrupted, or in other words, when you exit out of the game while it's saving. The environment, hardware, & software failures can also corrupt data. The best way to avoid this is to backup occasionally, & have several backups just to make sure, esp. if you're duping constantly.
Backups do not avoid the likelihood of corrupted data. They offer a way to recover to some point in time when the corrupted files are identified.