Number number isn't necessary necessary

Why do people say “PIN number” and “VIN number”?

Don't they know they’re really saying, “personal identification number number” and “vehicle identification number number”?

Really, it’s OK to just say PIN. Or VIN. No need to say “number” twice. Try it!

“I changed my PIN for the Pillow of the Month Club.”

“Charlie obliterated the VIN from his stolen tricycle.”