@nyergesdesign There's obviously been a mistake. Having the same departure and destination airport is strange anyway, but not completely unrealistic. However, the duplicate of the flight is a mistake. I guess, you like to fix it with the next update.
@pkorkala Navigate to (..\Steam\steamapps\common\) Tower 3D Pro\Extensions\Airfields\YMML and open gaandlocaltraffic.txt with a text editor (e.g. Windows Editor).
Search for 1DZD799, delete this line:
YMML, YMML, 10:58, SR2,1DZD799, 0, 0, 0, 1DZD799, ONE DELTA ZULU DELTA SEVEN NINE NINE
or change it to (or something else you like)
YMML, YWSL, 10:58, SR2,1DZR788, 0, 0, 0, 1DZR788, ONE DELTA ZULU ROMEO SEVEN EIGHT EIGHT
and save the file.
The next time you start the game the error should be fixed.