A CTLR ERRED IN CLRING THE WRONG ACFT FOR A DSCNT. THE B737-700 THEN RECEIVED A TCAS TA AND CLBED TO AVOID TFC.

Narrative

IN CRUISE AT FL390; ZID ISSUED US A DSCNT TO FL360; WHICH THE CAPT READ BACK AND SET. I VERIFIED AND REPEATED THE ALT AND INITIATED A DSCNT. AT ABOUT FL387; A TCAS TA APPEARED AT 1700 FT BELOW US AND CLBING AT ABOUT 11 MOVING TO 12 O'CLOCK POS. I BEGAN TO LEVEL THE ACFT OUT AND STOPPED THE DSCNT AT FL385 AND WE VISUALLY ACQUIRED THE TFC. THE CAPT THEN QUERIED ATC IF WE WERE INDEED SUPPOSED TO BE DSNDING TO FL360. ATC SAID; 'NO; DID YOU TAKE A CLRNC FOR SOMEONE ELSE?' THE CAPT REPLIED; 'NO; WE TOOK OUR CLRNC TO FL360.' SO WE CLBED BACK UP TO FL390. WE BELIEVE THE CTLR THEN REALIZED HIS ERROR BECAUSE WHEN HE GAVE US A FREQ CHANGE SHORTLY THEREAFTER; HE SAID THAT HE MAY HAVE GIVEN US A DSCNT WHEN HE MEANT TO GIVE A DSCNT CLRNC TO A DIFFERENT COMPANY FLT ON HIS FREQ. THERE WAS NEVER A TCAS RA; AND THE CLOSEST THAT THE ACFT GOT TO US WAS ABOUT 700 FT BELOW US AND AT THAT TIME WAS ABOUT 3 MI S OF OUR COURSE. HUMAN ERROR AND HVY WORKLOAD ON THE PART OF ATC ARE THE ONLY CONTRIBUTING FACTORS THAT I CAN ATTRIBUTE THIS LOSS OF SEPARATION TO AS THE CTR CTLR SEEMED TO BE BUSY AT THE TIME.