SIMILAR CALL SIGNS ACR X NON ADHERENCE TO ATC CLRNC DSCNT FROM ASSIGNED ALT.

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ACR X LEVEL 12000 FT MSL ON DSCNT INTO DETROIT METRO ARPT. PF MISUNDERSTOOD CLRNC ALT AND STARTED A DSCNT TO 11000 FT MSL. BEFORE 'I' COULD CORRECT THE PF HAD DSNDED TO 11700 FT MSL. I DIRECTED HIM TO CLB TO THE CLRNC ALT OF 12000 FT MSL. HE QUESTIONED THE ALT AND I RECOVERED THE ACFT; STARTED THE CLB BACK TO 12000 FT MSL AND RETURNED TO THE ACFT TO THE PF. THE PF OVER CORRECTED AND CLBED TO 12400 FT MSL. I PROMPTLY RELIEVED HIM FROM THE CTLS AND RETURNED THE ACFT TO 12000 FT MSL. OUR CLRNC ALT. SUPPLEMENTAL INFO FROM ACN 290306: ACR X LEVEL AT 12000 FT MSL CAPT READ BACK CLRNC FOR ANOTHER ACFT WITH SIMILAR CALL SIGN TO DSND TO 11000 FT. FO WAS FLYING AT SET 11000 FT IN ALT WINDOW; SELECTED FLT CHANGE. CAPT INQUIRED AS TO ACTUAL CALL SIGN FOR THE ALT CHANGE. WHEN IT WAS DISCOVERED WE HAD ANSWERED FOR ANOTHER ACFT; WE STOPPED OUR DOWN DEV JUST SHORT OF 300 FT WHILE CORRECTING OUR ALT WE DEVIATED UP JUST SHORT OF 500 FT HIGH. IT IS A BUSY TIME ON THE RADIOS DURING A 'HUB' ARR. ALL PLANES HAVE SIMILAR CALL SIGNS. AT THE TIME ALL ACFT WERE BEING GIVEN HDGS; AIRSPD AND ALTS; ALL AT ONCE. WE SHOULD HAVE LISTENED BETTER.