A BOEING 757 FLC ACCEPTED A CLRNC INTENDED FOR ANOTHER ACFT AND BEGAN DSCNT NEAR ORD.

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WE WERE ON THE OKK ARR TO ORD AT 11000 FT. I WAS FLYING THE ACFT ON AUTOPLT. I WAS MOMENTARILY DISTRACTED WITHIN THE COCKPIT JUST AS APCH ISSUED A DSCNT CLRNC. THE FO READ BACK 'ACR XX DSND TO 4000 FT' AND A TURN AND ALTIMETER SETTING. THE CTLR DID NOT CHALLENGE THE ACCURACY OF THE READBACK. THE FO AND I CONFIRMED 4000 FT SET IN THE ALT WINDOW AND WE STARTED DOWN. PASSING 6400 FT THE CTLR GAVE A 20 DEG HDG CHANGE AND SAID MAINTAIN 7000 FT. I DISCONNECTED THE AUTOPLT AND IMMEDIATELY INITIATED A CLB TOWARD 7000 FT. THE FO STATED TO APCH THAT WE WERE RETURNING TO 7000 FT; BUT THAT HE HAD GIVEN US 4000 FT BEFORE. PRIOR TO ARR AT 7000 FT THE CTLR AGAIN CLRED US TO 4000 FT. THERE WERE NO TCASII ALERTS FOR TFC NOR DID THE CTLR SEEM TOO CONCERNED. I KNOW THE WORKLOAD IN THE HIGH VOLUME CITIES HAS BECOME CRITICAL; BUT I PERSONALLY THINK CTLR VERIFICATION OF READBACK CLRNCS IS ABSOLUTELY ESSENTIAL TO TFC SEPARATION AND SAFETY. I ALSO BELIEVE BOTH PLTS SHOULD ABSOLUTELY BE SURE OF THE CLRNC PRIOR TO ACTING ON IT. SUPPLEMENTAL INFO FROM ACN 451537: THE APCH CTLR DID NOT CHALLENGE THE ACCURACY OF THE READBACK. THE CAPT AND I BOTH CONFIRMED 4000 FT SET IN OUR ALT WINDOW AND WE STARTED DOWN. PASSING 6400 FT THE CTLR GAVE A 20 DEG HDG CHANGE AND SAID MAINTAIN 7000 FT. I THINK CTLR VERIFICATION OF READBACK CLRNCS IS ABSOLUTELY ESSENTIAL TO TFC SEPARATION AND SAFETY.