The same frequency is listed twice because the services provided on that frequency are mutually exclusive. When one service is available, the other cannot be.
When the airport has an operational control tower, airport traffic control service is available on the Tower frequency:
AIRPORT TRAFFIC CONTROL SERVICE- A service provided by a control tower for aircraft operating on the movement area and in the vicinity of an airport.
(See MOVEMENT AREA, TOWER, ICAO term AERODROME CONTROL SERVICE.)
This service is provided to ensure safe spacing between aircraft operating on the airport surface, in particular the runways. Some airborne separation services may also be provided between IFR/IFR, IFR/VFR, and even VFR/VFR traffic depending on the airspace classification.
When the airport does not have an operational traffic control tower, this service is not available. Instead, pilots use the Common Traffic Advisory Frequency to self-announce their position and intentions, which allows operations to be carried out in a safe manner.
COMMON TRAFFIC ADVISORY FREQUENCY (CTAF)- A frequency designed for the purpose of carrying out airport advisory practices while operating to or from an airport without an operating control tower. The CTAF may be a UNICOM, Multicom, FSS, or tower frequency and is identified in appropriate aeronautical publications.
(See DESIGNATED COMMON TRAFFIC ADVISORY FREQUENCY (CTAF) AREA.)
(Refer to AC 90-66, Non-Towered Airport Flight Operations.)
This is not a separation service, but a frequency provided to aid pilots in their see-and-avoid and safe sequencing responsibilities. The Federal Aviation Regulations still apply and right-of-way rules must still be followed, even if an aircraft is not transmitting on the CTAF.
At many airports with part-time control towers, the main Tower frequency and the CTAF share the same frequency, indicated on the Sectional like this:
CT - 118.6 ★ ©.
But this does not imply that they share the same function, which is why the Chart Supplement both frequencies separately. The star symbol indicates that the Tower frequency is only in operation some of the time, and when it is not the "C" symbol indicates that the CTAF is 118.6 as well; when the Tower is open there is no CTAF. This configuration is very common at fields with part-time towers but it may not always hold true.
So it would be incorrect to say that you "contact the tower on CTAF."
Instead you contact the Tower on the Tower frequency, and utilize the CTAF if the Tower is closed.