“PIN Number” — why do we say it?
Assume we abbreviate "Orthogonal Multi-user Channel" as OMC. Is it correct to use the phrase "OMC channel" since channel is already contained in OMC?
This is commonly referred to as RAS Syndrome, or Redundant Acronym Syndrome Syndrome. Other examples of this are ATM Machine (Automated Teller Machine Machine) and PIN Number (Personal Identification Number Number).
This is not technically correct, but it happens pretty often.