As far as I know--codes mailed electronically or snail mail are intended for recipients and adult members of the same househould who share the same address.
My mother has traveled with me before--so she's forever on my record. We got a postcard with her name on it--but we can use it for ourselves b/c it is our address.
Not sure how you used a code. I have experienced some lax handling of our ressie by the CM's when we check in. We book Fla Resident specials a lot--so they don't check our ID all the time. Perhaps that is what happen when you used someone else's code before.