A camera magazine has recently published an alert, over fake Hoya filters being manufactured in China, and apparently being marketed on certain websites at a price somewhat cheaper than the real thing. The magazine does not name the websites, presumably for legal reasons. These filters are allegedly of very poor quality, both optically and construction wise.
This follows in the wake of fake chargers for mobile phones (and possibly cameras) etc. also eminating from China.
It seems to be the case that any camera accesories originating from China and being sold at suspiciously cheap prices, should be regarded with extreme caution before considering purchase.
Moral......If it looks too good to be true, it almost certainly is.