Air Traffic Control Fully Certified
Confusion; Other / Unknown; Situational Awareness
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
ATC Equipment / Nav Facility / Buildings; Company Policy
A B757-200 was departing Runway 25R. The LC2 assist was unable to rundown the departure via automation; had to verbally run down aircraft. I investigated as to the cause of the 'dup id' message and found that there was another flight with same call sign; but as an arrival to LAX. The 'T' affixed to the end of the callsign led me to believe the departure was a continuation of this arrival; and that the arrival had already landed and the RADAR track was in coast mode; not uncommon at LAX. I removed the arrival control by using the OK function. The departure rundown was now completed via automation. Later I found out that the arrival was still inbound; and due to my actions; the data tag had been lost; but the RADAR track was still displayed. In removing the arrival control track; I created the same situation I was trying to prevent; i.e. have an aircraft airborne with no associated data tag. In this case; an un-tagged departure would have been better than an un-tagged arrival. The first change would be to discuss with the company why they used a 'T' suffix on two callsigns; when the use of the suffix is to resolve confusion to same callsign events (like this one). A better fix would be to use two different suffixes; a 'Q' or 'J' etc.
LAX Controller described a confused event when an automated departure run down to the TRACON failed because of an apparent 'dupe ID;' the reporter's corrective actions resulted in an arrival aircraft losing data block information.