I am not sure why you would be buying the older 50mm model, considering the newer one is better in almost every way. However, if you have bought an older 50mm F/1.8 II recently, you may want to take a look at it to confirm that you have a legitimate Canon lens and not one of these fakes.
To do this, take the lens off your camera, or remove the rear lens cap. Canon says to look for the Canon branding along the lens mount. On the real version of the Canon 50mm F/1.8 II you can see there is a space between Canon and Inc, whereas on the counterfeit version there is no space between the two words.
According to the Canon report, the existence of these fake lenses was discovered when some of them were sent to a Canon repair center to be repaired. It was then discovered that the lenses were in fact not genuine Canon products.
You can find more information on Canon’s website in regards to how to look for counterfeit lenses and cameras, here. If you happen to discover that a lens or camera in your collection is not genuine, you can contact Canon about that, here.