I haven't seen the option to set up write caching.I don't know your NAS but do you by chance have write caching running without a UPS? This is a no no. When power goes out it corrupts. Set write caching to write through. It is slower but much safer. Or use a UPS for your NAS. The same thing holds for Windows.
I bought a UPS few weeks ago but it doesn't change how files are being handled by the NAS. Or at least, I haven't seen any change.
But the duplicate files appeared before I set up the UPS. I bought the UPS after the power breakdown, as a matter of fact.