APCH CTLR AT ATL CONFUSES SIMILAR CALL SIGNS OF 2 SAME COMPANY ACFT IN TRAIL ON DOWNWIND LEG. THE LEAD COMPANY APPARENTLY CONFLICTED WITH A DC9 ON BASE LEG RESULTING IN A LOSS OF SEPARATION AND AN NMAC.

Narrative

I WAS THE CAPT AND PNF OF ACR FLT X ARRIVING IN ATL. WE WERE BEING VECTORED FOR A VISUAL APCH TO RWY 26R IN ATL AT THE SAME TIME AS ACR FLT Y. SEVERAL TIMES THROUGHOUT THE VECTORING ON DOWNWIND; IT WAS APPARENT THAT THE CTLR WAS CONFUSING THE FLT NUMBERS OF THE 2 COMPANY ACFT. DUE TO THE HVY ARR TFC; INSTRUCTIONS ARE OFTEN GIVEN WITH LITTLE OR NO CHANCE OF RESPONSE. AS WE WERE BEING VECTORED ON A R DOWNWIND TO RWY 26R AT 5000 FT; WE NOTICED ANOTHER ACFT AHEAD OF US BEING VECTORED ON A R BASE AT OR NEAR OUR ALT. IT WAS AN ACR Z DC9 AND HIS FLT PATH WOULD DEFINITELY BE A CONFLICT WITH OURS. THE CTLR HAD OUR POS/CALL SIGN CONFUSED WITH THE POS AND CALL SIGN OF OUR COMPANY ACFT ACR X (US) AND ACR Y AND THEREFORE HAD GIVEN VECTORS CAUSING THE CONFLICT. WE HAD THE CONFLICTING ACFT; THE ACR Z DC9; IN VIEW VISUALLY ON OUR TCASII DISPLAY AS THIS DEVELOPED AND INITIATED A SLIGHT TURN APPROX 15 DEGS L TO PUT OUR ACFT BEHIND THE FLT PATH OF THE ACR Z ACFT. AT ABOUT THE SAME TIME THE ACR Z ACFT INITIATED A CLB AND STATED HE WAS CLBING IN RESPONSE TO AN RA. THE CTLR WHO WAS OBVIOUSLY CONFUSED ISSUED AVOIDANCE INSTRUCTIONS TO THE ACR Z ACFT AS WELL. THE WHOLE PROB COULD HAVE BEEN AVOIDED IF COMPANY WOULD HAVE MADE SURE THAT SUCH SIMILAR CALL SIGNS WERE NOT ASSIGNED TO ACFT ARRIVING AT THE SAME ARPT AT THE SAME TIME. IN IMC CONDITIONS THIS MAY HAVE BEEN A VERY A CLOSE CALL INSTEAD OF JUST A SIMPLE AVOIDANCE MANEUVER.