I dun have this problem. I only receive 1 notification for SMS.
For assigning ringtones to contacts, this option is not available if your contacts are not phone contacts, eg contacts synced with gmail. The only way to assign ringtone to these contact is to create a phone contact and link the 2 together. Then you can assign the ringtone to the phone contact. This works for me.
Samsung Galaxy Note II LTE N7105
Dopod 838 Pro -> Asus P750 -> HTC Touch Pro -> HTC Touch Pro2 -> Samsung Galaxy S -> Samsung Galaxy SII -> Asus Padfone -> Samsung Galaxy Note II LTE -> ??