Small Aircraft; Low Wing; 2 Eng; Retractable Gear
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 10
Confusion; Distraction; Troubleshooting
ATC Issue All Types
Air Traffic Control Separated Traffic
ATC Equipment / Nav Facility / Buildings; Procedure
CCR is a level 5 VFR tower.  Controlling RADAR approach facility is SUU RAPCON.  RADAR information is an ARTCC site 30 miles west processed through ARTS IIIE RDP/FDP at Northern California TRACON.  CCR's RADAR processing does not interface with SUU's RADAR equipment. 95% of the time we cannot see their data blocks and they can't see ours.  When inbounds have been initially tagged up by Center or NCT Approach; we do receive FDIO strips on the inbound practice approaches.  Method of transfer of control of IFR/VFR approaches (all from the North) is via coordination with squawk code. The following is a previously reported known issue.Aircraft X was coordinated inbound from SUU on a GPS19R VFR practice approach.  CCR had an FDIO printed strip that matched the coordinated inbound call sign and squawk.  I was watching for the inbound target to join my final approach course; which would normally be displaying only a * (splat); altitude; and speed.  I suddenly realized there was a data block descending and heading my way.  It displayed Aircraft Y; destination; PAO (Palo Alto). I checked the squawk corresponding to the data block.  It was [XXXX]; the same as the inbound coordinated and printed out on Aircraft X.  I called SUU RADAR and they advised the data block showing on their RADAR was Aircraft X on the same target that mine displayed as Aircraft Y.It has been determined that this occurs when Northern California TRACON works an aircraft earlier in the day.  They terminate RADAR services on the aircraft; but fail to terminate control on the data block.  After that flight drops off RADAR; instead of dropping out of NCT's FDP; it remains there floating around; even though it has dropped out of the ARTCC ERAM FDP.  ERAM believes it available and assigns it to another flight; but on any screen associated with NCT's FDP (such as ours); it shows up with the old inactive call sign.This fortunately only happens about once every 3 or 4 weeks; but it can be very distracting and discomforting when it does; and in busy traffic; could become a safety issue.  The targets that drop off NCT's RADAR should be programmed to drop out of the system after a period of time.
CCR FLM describes a problem with the Radar system that shows duplicate beacon codes at times.  Aircraft that are not removed from the system correctly tag up later in the day on another aircraft.