A DC9 FLC CONTINUES THEIR DSCNT TO A PREVIOUSLY CLRED TO ALT AFTER AN AMENDED ALT CHANGE HAD BEEN RECEIVED FROM ZOB NEAR COFAX INTXN; PA.

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INBOUND TO COFAX INTXN ON BOJID ARR TO PHL; CLE GAVE CLRNC FROM FL310 TO FL280 AND TO CROSS COFAX AT FL250. SHORTLY THEREAFTER; CLRNC TO AMEND ALT TO FL290 FROM CLEVELAND. THE CAPT (PF) CONTINUED AFTER A BRIEF LEVELOFF AT FL290 TO CONTINUE DSCNT SO AS TO CROSS COFAX AT FL250. AS WE CROSSED FL275; ZOB ASKED IF WE HAD RECEIVED THE FL290 CLRNC; AND GAVE A HDG CHANGE TO R FOR TFC AVOIDANCE. BOTH PLTS UNDERSTOOD THE FL290 CLRNC. AS PNF; I DID NOT QUESTION CAPT'S (PF) ACTION TO CONTINUE THE DSCNT AS A SIMILAR ISSUE OCCURRED PREVIOUSLY ON THE TRIP (PRIOR DAY) AND I INFERRED FROM THAT SIT THE CAPT'S POS IN THIS CASE -- WHICH I WAS UNSURE OF. I HAD PLANNED TO INVESTIGATE THE AIM WHEN I HAD ACCESS TO THE AIM FOR REF AT THE CONCLUSION OF THE TRIP. CTR GAVE AVOIDANCE VECTORS TO US AND THE OTHER ACFT AND WE CONTINUED DSCNT TO FL250. NO TCASII RA/TA WAS RECEIVED AND WE NEVER FELT THREATENED BY THE OTHER ACFT. THE CAPT ASKED CTR IF THIS WAS A 'PROB.' THE CTR SAID NO IT WASN'T. WE WERE THEN HANDED OFF TO ZNY WHO REITERATED CLEVELAND'S STATEMENT THAT THERE WASN'T A PROB. THE CAPT LATER STATED TO ME THAT HE FELT A SUPVR HAD GOTTEN INTO THE COM EXCHANGE AT THE CTR AND SUGGESTED THAT THIS WAS A CTR OPERROR. I HAVE SINCE HAD MY DOUBTS AND BELIEVE THAT THIS SHOULD BE RPTED AS A CREW ERROR. THIS WAS THE LAST LEG OF 5 DAYS ON DUTY. I HAD GOTTEN VERY LITTLE SLEEP THE PREVIOUS NIGHT DUE TO NOISE AROUND THE HOTEL. FIRST LEG OF 3 THAT DAY; ON DUTY AT EARLY MORNING.