ACR LGT DESCENDED THROUGH ASSIGNED ALT AFTER READING BACK INCORRECT LOWER ALT WITHOUT ATC CORRECTION.

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I WAS FLYING AND THE CAPT WAS TALKING TO ATC. WE WERE CLRD TO 4000' BY APCH CTL AND SLOW TO 170 KTS AT 4000'. THE PROB FIRST BEGAN WHEN; BEFORE REACHING 4000'; I HEARD A CLRNC TO 1700'. I WAS NOT SURE WHO IT WAS FOR BECAUSE I HAD MISSED THE CALL SIGN. BUT; I DISTINCTLY HEARD THE CAPT READ BACK OUR CALL SIGN AND CLRD TO 1700' LEAVING 5000'. THE CTLR RESPONDED; 'ROGER.' I REMEMBER LISTENING FOR THE CTLR'S RESPONSE BECAUSE I DID HAVE A QUESTION ABOUT THE CLRNC. MY DOUBT WAS GONE. A SECOND READBACK OF 1700' CLRD ALT TOOK PLACE WHEN THE CTLR AGAIN ASKED US TO SLOW TO 170 KTS. CAPT SAID; 'ROGER; SLOW TO 170 KTS THEN DOWN TO 1700'.' JUST BEFORE WE LEVELED AT 1700' THE CTLR SAID TO US; 'CHK YOUR ALT.' CAPT SAID LEVELING 1700' AND I READ BACK 1700'. CTLR SAID; '1700' ALT WAS FOR ANOTHER ACFT; BUT YOU CAN STAY AT 1700'.' TO PREVENT THIS FROM HAPPENING AGAIN; ANYTIME I HAVE ANY DOUBT ABOUT A CLRNC I WILL USE THE WORD CONFIRM IN MY READBACK TO INCREASE THE CTLR'S ATTN TO MY READBACK. SUPPLEMENTAL INFORMATION FROM ACN 80109: PASSING 5000' I THOUGHT I HEARD; 'COMPANY XAY; DES TO 1700.' I READ BACK; 'COMPANY XAY DSNDING TO 1700'; LEAVING 5000'.' AS WE APCHED 4000' THE CTLR SAID; 'COMPANY XAY SLOW TO 170 KTS AT 4000'.' I REPLIED; 'ROGER; SLOW TO 170 KTS AT 4000'; DSNDING TO 1700'; COMPANY XAY.' NO RESPONSE WAS HEARD. AT ABOUT 2400'; THE CTLR SAID; 'COMPANY XAY; CHK ALT.' I REPLIED; 'LEAVING 3 FOR 1700'; I PREVIOUSLY READ BACK MY CLRNC TO 1700'.' THE CTLR RESPONDED; 'ROGER; DES NOW TO 1700'; THAT CLRNC WAS FOR ACR B XBY.' THE COPLT; WHO WAS FLYING THE LGT; TOLD ME THAT THERE WAS SOME QUESTION IN HIS MIND ABOUT THE CLRNC; BUT ALL DOUBT WAS REMOVED WHEN I READ BACK THE CLRNC A SECOND TIME. PERHAPS IF THE F/O VOICED HIS DOUBTS THE FIRST TIME IT COULD HAVE BEEN CLARIFIED. I DO FEEL THE CTLR SHOULD HAVE CAUGHT THE DISCREPANCY ON ONE OF MY READBACKS.