Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 9
Distraction; Confusion; Human-Machine Interface; Situational Awareness; Training / Qualification; Troubleshooting
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
General None Reported / Taken
Procedure
Aircraft X had check on to Lindbergh tower's local control position and was operating in the San Diego Bravo airspace within 1 mile to 1 1/2 miles south of the final approach course. The position tag (S) dropped. Local controller noticed it; attempted to retag but it indicated 'dup ID'. SAN Local control had to retag Aircraft X using a different ID. When SAN LC check the call sign it indicated it was now being used by 2T. 2T is LAX Tower. Prior report was filed for the same occurrence and the practice of LAX Twr dropping SAN Tags to be used by them. It is inexcusable for any controller to think it is okay to remove someone else's flight plan. Ever! I was notified by the CPC that LAX was still doing this reckless behavior. LAX doing this with a carrier tag between LA Arrival and LAX Twr was the topic of a video in recurrent training. Why is it still occurring? I don't know how to be clearer. It has to stop!I recommend individual; face-to-face briefing on the proper etiquette of Air Traffic Control. How one should not remove flight plans that do not belong to that facility! The last response from LAX was that 'This conduct would no longer be condoned'. What?  When would it ever be condoned? SCT originates our LOAs. SCT could keep a master list of all call sign usage to alleviate dual use. We have tours that operate in the SAN Bravo. SCT San Diego area has sky diving. We were both attempting to use 'SKY' as an identifier when talking to our individual aircraft company's. We coordinated with each other to use different IDs; but those call signs used could be on a master list to let others know it isn't available.
SAN Tower Front Line Manager reported of duplicate aircraft identifications in the SAN and LAX Area.  Someone was attempting to use a call sign; saw that it was being used; but dropped the data tag anyways.