AN ACR ACFT LEFT ITS ALT WITHOUT CLRNC. THE CLRNC WAS FOR ANOTHER ACFT.

Narrative

ACR W WAS 10 MI INTRAIL OF ACR X; ALSO AT 10000 FT; PROCEEDING OVER THE SAME DEP GATE (W). I CALLED TFC TO ACR W AS 10 O'CLOCK; 10 MI AT 11000 FT AND 10 O'CLOCK; 12 MI AT 12000 FT; BOTH NEBOUND. I ADVISED ACR W I WOULD HAVE HIGHER WHEN HE WAS CLR OF BOTH ACFT. MY VERY NEXT XMISSION WAS TO ACR X TO CLB TO 13000 FT AND TO CONTACT ZOB. I VERIFIED THE ALT AND FREQ AS BEING CORRECT BUT DID NOT CATCH THE CALL SIGN. ABOUT 20 SECONDS LATER; ACR X REQUESTED HIGHER. I CLBED AND SWITCHED ACR X A SECOND TIME (OR SO I THOUGHT); THEN LOOKED AROUND TO SEE WHO ACKNOWLEDGED THE FIRST TIME. I SAW ACR W LEAVING 10200 FT; 8 MI IN FRONT OF THE SMT Y AT 11000 FT AND 10 MI IN FRONT OF THE MLG Z AT 12000 FT. ALTHOUGH I DID NOT CLARIFY THE CORRECT CALL SIGN THAT READ BACK THE FIRST CLRNC; I CANNOT IMAGINE WHY ANY PLT WOULD CLB WITHOUT QUESTION WHEN HE HAD JUST BEEN ISSUED 2 CONVERGING TARGETS AT ALTS ABOVE HIM. THIS ACFT WAS TCASII EQUIPPED AND THE PLT COULD ALSO HAVE USED THIS SYS TO CHK THE TFC. SUPPLEMENTAL INFORMATION FROM ACN 255414: WE RECEIVED WHAT WE THOUGHT WAS CLRNC FOR US (ACR W) TO CLB TO 13000 FT AND CONTACT ZOB. WE WERE INFORMED BY OUR UNION SAFETY CHAIRMAN THAT WE HAD ACCEPTED THE 13000 FT CLB AND FREQ CHANGE FOR ANOTHER FLT; ACR X; WITH A SIMILAR SOUNDING CALL SIGN. DETROIT DEP DID NOT CATCH OUR READBACK AND WE WERE NOT AWARE OF OUR MISTAKE. WE WERE AT A PLACE TO RECEIVE THE CLRNC WE TOOK BY MISTAKE. CORRECTIVE ACTION: REDUCE; IF NOT ELIMINATE; SIMILAR SOUNDING CALL SIGNS. REDUCE CTLR WORKLOAD SO THAT THEY CAN HEAR READBACKS.