Narrative:

Light transport X departed ord squawking the code assigned to company flight light transport xy. This caused light transport X's data block to automatically acquire on light transport xy's target. Believing that I was talking to light transport X, I assigned him a 180 degree heading. Since that was the wrong call sign, the aircraft never turned. Complicating the situation, the real light transport xy was also flying in my sector, so of course all of my xmissions were being acknowledged. After I assigned light transport xy a 180 degree heading, I turned large transport Z to a 270 degree heading, which would have taken him behind the light transport X flight. However, since the light transport X flight had never turned, approved sep was lost. Since conditions were VFR, the 2 aircraft had each other in sight. When we checked the tapes, the crew of light transport X had been assigned the proper beacon code, and acknowledged the code properly, but did not squawk it. This could have been prevented by the crew squawking the right beacon code, by the tower controller noticing on his radar display that the data block had acquired on the wrong aircraft, or by my noticing sooner that the light transport X flight had not turned when I assigned the heading. Traffic at the time was moderate, and complexity was moderate to difficult.

Related reports:

Google
 

Original NASA ASRS Text

Title: AN ACFT USED A TRANSPONDER CODE ASSIGNED TO ANOTHER ACFT WHICH RESULTED IN LESS THAN STANDARD SEPARATION.

Narrative: LTT X DEPARTED ORD SQUAWKING THE CODE ASSIGNED TO COMPANY FLT LTT XY. THIS CAUSED LTT X'S DATA BLOCK TO AUTOMATICALLY ACQUIRE ON LTT XY'S TARGET. BELIEVING THAT I WAS TALKING TO LTT X, I ASSIGNED HIM A 180 DEG HDG. SINCE THAT WAS THE WRONG CALL SIGN, THE ACFT NEVER TURNED. COMPLICATING THE SITUATION, THE REAL LTT XY WAS ALSO FLYING IN MY SECTOR, SO OF COURSE ALL OF MY XMISSIONS WERE BEING ACKNOWLEDGED. AFTER I ASSIGNED LTT XY A 180 DEG HDG, I TURNED LGT Z TO A 270 DEG HDG, WHICH WOULD HAVE TAKEN HIM BEHIND THE LTT X FLT. HOWEVER, SINCE THE LTT X FLT HAD NEVER TURNED, APPROVED SEP WAS LOST. SINCE CONDITIONS WERE VFR, THE 2 ACFT HAD EACH OTHER IN SIGHT. WHEN WE CHKED THE TAPES, THE CREW OF LTT X HAD BEEN ASSIGNED THE PROPER BEACON CODE, AND ACKNOWLEDGED THE CODE PROPERLY, BUT DID NOT SQUAWK IT. THIS COULD HAVE BEEN PREVENTED BY THE CREW SQUAWKING THE RIGHT BEACON CODE, BY THE TWR CTLR NOTICING ON HIS RADAR DISPLAY THAT THE DATA BLOCK HAD ACQUIRED ON THE WRONG ACFT, OR BY MY NOTICING SOONER THAT THE LTT X FLT HAD NOT TURNED WHEN I ASSIGNED THE HDG. TFC AT THE TIME WAS MODERATE, AND COMPLEXITY WAS MODERATE TO DIFFICULT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.