SMA Y NON ADHERENCE TO ATC INSTRUCTION HAD NMAC WITH SMA X IN TFC PATTERN. SEE AND AVOID CONCEPT.

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I WAS WORKING LCL CTL POS WHEN AN SMA X RPTED THAT HE WAS INBOUND FOR LNDG FROM PERIMETER MALL; WHICH IS NW OF PDK ARPT. I INSTRUCTED THE SMA X TO ENTER A L DOWNWIND AND ISSUED TFC ABOVE AND BEHIND HIM DSNDING FROM 5400 FT MSL. SMA X WAS DSNDING FROM 2400 MSL TO 2000 MSL. A SHORT TIME LATER; SMA Y RPTED INBOUND FROM THE NW. AT THAT TIME; I INSTRUCTED THE SMA Y TO ENTER A WIDENED OUT 3 MI L DOWNWIND TO AVERT THE SMA X TFC BELOW AND IN FRONT OF HIM; WHICH I ADVISED HIM OF. THE SMA Y NEGLECTED TO FOLLOW MY INSTRUCTIONS AND PROCEEDED TO ENTER A NORMAL L DOWNWIND ABOUT 500 FT ABOVE THE SMA X. ONCE I OBSERVED BOTH ACFT IN THE L DOWNWIND; I INSTRUCTED SMA Y TO MAKE A R 360 DEG TURN AND FOLLOW A THIRD ACFT BEHIND HIM. AS I OBSERVED THE TURN; I REALIZED SMA Y WAS AT A VANTAGE POINT WHERE HE COULD SIGHT THE SMA X; SO I TOLD HIM TO REMAIN IN THE DOWNWIND AND FOLLOW THE SMA X OFF HIS L. THE SMA X DID HAVE SMA Y IN SIGHT; SO I CHANGED HIS RWY TO RWY 2R AND CLRED SMA Y TO LAND ON RWY 2L. I BELIEVE THERE WAS A STUDENT FLYING SMA Y WITH AN INSTRUCTOR ON BOARD.