MLG TOOK TURN INTENDED FOR COMPANY FLT WITH SIMILAR CALL SIGN AND BOTH CTLR AND CAPT REPORTER CAUGHT PROBLEM AT SAME TIME. NO SEPARATION LOST.

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MY COPLT AND I (MLG ABC) HAD JUST BEEN CLRD FOR TKOF FROM ATL; RWY 9L AND INSTRUCTED TO FLY HDG 095 DEGS. TKOF WAS NORMAL WITH ME (CAPT) HAND FLYING OUR MLG (ABC). TWR INSTRUCTED US TO SWITCH OVER TO DEP CTL. MY F/O MADE THE SWITCH AND CONTACTED ATL DEP CTL (125.0) AND CHKED IN. 'ATL DEP; MLG (ABC) WITH YOU; 2000 CLBING TO 10000.' DEP REPLIED; 'ROGER MLG (ABC); TURN LEFT 360 DEGS.' MY F/O ACKNOWLEDGED THE TURN. I IMMEDIATELY TOLD MY F/O THAT THAT WAS A STRANGE TURN FOR US (DEPARTING RWY 9L). AS I STARTED A TURN TO 360 DEGS; I SAW THERE WAS AN LGT ON DEP FROM THE N RWY. THE LGT WAS OFF THE GND; EVEN (ALONG SIDE OF) US BUT MUCH LOWER. I STOPPED OUR TURN AT HDG OF 080 OR 085 DEGS AND TOLD MY F/O OF THE OTHER ACFT AND ASKED HIM TO VERIFY THE TURN WITH DEP. AT THIS TIME; DEP CAME BACK ON AND TOLD US TO FLY THE HDG ASSIGNED BY THE TWR (095 DEGS); THEN IMMEDIATELY SAID; 'MLG (ABC); FLY HDG 120 DEGS; VERIFY CLBING TO 10000'.' WE ACKNOWLEDGED AND I ASKED THE F/O TO FLY THE ACFT; THAT I WANTED TO TALK TO DEP. DEP THEN TOLD US THAT WE HAD TAKEN A TURN FOR LGT (BBC). HE THEN MADE SEVERAL OTHER TRANSMISSIONS. I CALLED HIM AND TOLD HIM THAT I WANTED TO TALK TO HIM WHEN HE GOT A FREE MOMENT. HE MADE SEVERAL OTHER TRANSMISSIONS AND THEN CAME BACK AND ASKED WHAT I WANTED. I WENT OVER THE SEQUENCE OF EVENTS WITH HIM AND HE INFORMED ME THAT WE HAD TAKEN THE CALL FOR LGT BBC AND HE INDICATED NO ACKNOWLEDGEMENT OF OUR READBACK. WE THEN SWITCHED FREQ AND THAT WAS THE END OF THE INCIDENT. WE NEVER CAME CLOSE TO THE OTHER ACFT AND I HAD VISUAL CONTACT UNTIL AFTER WE WERE HDG 120 DEGS. BOTH MY F/O AND I WERE WEARING HEADSETS AND CLEARLY HEARD ALL TRANSMISSIONS. I BELIEVE THAT WHEN MY F/O CALLED IN 'MLG (ABC)' HE PROGRAMMED THE CTLRS MIND (WHO WAS GETTING READY TO CALL LGT (BBC); AND HE SUBSEQUENTLY GAVE US THE TURN. SUPPLEMENTAL INFO FROM ACN 81241: I (F/O; PNF) CHKED IN WITH 'DEP; MLG (ABC) 2 FOR 10.' DEP'S RESPONSE WAS; 'MLG (ABC) ROGER; TURN LEFT TO 360 DEGS.' I READ BACK 'ROGER; MLG (ABC) LEFT TO 360.' (WE WERE AT 1000' AGL; OFF THE END OF 9L AT ATL.) BECAUSE OF OUR LOCALE THE CAPT IMMEDIATELY SAID SOMETHING LIKE; THAT'S A STRANGE REQUEST; THERE'S AN AIRPLANE DOWN THERE TOO; CHK AND SEE IF HE REALLY WANTS US TO DO THAT. DURING THIS TIME OUR HDG CHANGED ONLY ABOUT 5 DEGS; AT WHICH POINT THE CAPT STOPPED THE TURN. JUST AT THAT MOMENT AND JUST BEFORE I HAD A CHANCE TO KEY THE MIC AND ASK FOR VERIFICATION OF INSTRUCTIONS; THE CTLR TRANSMITTED TO US TO RESUME THE HDG THAT TWR HAD GIVE US (095 DEGS) AND VERIFY THAT WE WERE CLBING TO 10000'. AT THE REQUEST OF THE CAPT A CONVERSATION FOLLOWED. THE CTLR FELT THAT WE HAD TAKEN A TURN WHICH WAS ORDERED TO FLT LGT (BBC). BOTH THE CAPT AND I HEARD (ABC) AND I READ BACK 'MLG (ABC) LEFT TO 360 DEGS.' LGT (BBC) WAS ON ANOTHER FREQ ACCORDING TO THE DEP CTLR. PERHAPS WHEN I CHKED IN IT SORT OF PROGRAMMED THE DEP CTLR'S MIND TO (ABC) INSTEAD OF (BBC). HE ROGERED OUR CHK-IN AND GAVE THE TURN ALL IN THE SAME TRANSMISSIONS AND UNDER THE SINGLE CALL SIGN; AND HE DIDN'T CATCH HIS ERROR. AFTER MY READBACK FOR THAT MOMENT I BELIEVE HE HAD US CONFUSED WITH LGT (BBC). THERE WAS NO DANGER BECAUSE THE CAPT IMMEDIATELY SENSED THAT SOMETHING WAS NOT RIGHT AND THEREFORE ONLY CHANGED OUR HDG ABOUT 5 DEGS. ALSO; THE AIRPLANE ON THE N SIDE AND ABEAM OF US WAS WELL BELOW US.