A CL65 FLC LEAVES THEIR LAST ASSIGNED ALT WHEN CLRED BY THE CTR CTLR TO 16000 FT; 'NOW.' 2 SIMILAR SOUNDING CALL SIGNS ON FREQ; CTR ISSUED THE WRONG FLT THE CLRNC. ZNY; NY.

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ON OUR 6TH LEG; 12 1/2 HRS DUTY; 8TH FLT HR; 1 HR GND STOP AT LGA; NEW FO (R SEAT). FO CHKED IN WITH ZNY AS FHE. ABOUT 30 SECONDS LATER; CTR XMITS; 'FHE; I NEED YOU TO START YOUR DSCNT TO 16000 FT NOW;' WITH SOME URGENCY. MY FO WAS MISSING CALLS ALL DAY AND WAS OBVIOUSLY TIRED (I WAS; TOO). I HOPPED ON THE RADIO AND REPLIED 'YOU NEVER GAVE FHE A DSCNT; WE'RE STARTING DOWN NOW.' I BEGAN A RAPID DSCNT TOWARDS 16000 FT; IE; FLT IDLE/3000 FPM. THIS IS MY COMPANY'S STANDARD DSCNT PROFILE. AT APPROX 19500 FT CTR REPLIED; 'EHE; CLRED TO 16000 FT; ALTIMETER ....' EHE REPLIED 'ROGER; DSND TO 16000 FT.' THIS WAS THE FIRST INDICATION THAT EHE WAS ON OUR FREQ. I STOPPED THE DSCNT ABRUPTLY AND CALLED ATC TO CLARIFY THAT HE HAD GIVEN US A DSCNT TO 16000 FT. HE REPLIED; 'RECLRED TO FL200.' I WAS ALREADY CLBING BACK TO FL200. MOMENTS LATER; ATC APOLOGIZED; HE HADN'T REALIZED WE HAD CHKED IN. HE THEN INFORMED US AND EHE THAT BOTH WERE ON THE FREQ. FACTORS: A) POOR SCHEDULING PRACTICES. 1) MY COMPANY SHOULD NOT SCHEDULE 13 HR DUTY DAYS. WE WERE ON OUR 6TH LEG; AND 12 1/2 HR DAY. 2) FHE AND EHE SHOULD NOT BE COMING FROM SAME DIRECTION AT THE SAME TIME OF DAY BACK TO THE HUB. (LATER ON 2 FLTS WERE ALL ON APCH.) B) ATC DID NOT TELL US TO EXPECT EHE SIMILAR CALL SIGN ON FREQ. C) ATC SHOULD NOT MAKE AN URGENT CALL TO DSND TO AN ACFT WHOM HE KNOWS HAS A SIMILAR CALL SIGN. D) EHE (OTHER ACFT) DID NOT REPLY TO URGENT DSCNT CALL. (HE ALSO MISSED OTHER CALLS FURTHER TOWARD HUB.) 3) WE WERE IN A BUSY MODE OF FLT -- 120 MI; 20 MINS OUT FROM LNDG. MY COMPANY DOES NOT HAVE ACARS ON MY FLEET. WE MUST MONITOR 2 FREQS. ATC AND COMPANY ATIS MUST BE LISTENED TO AND HAND WRITTEN. EVEN THE ACFT WHICH HAVE ACARS CANNOT GET ATIS BECAUSE AIRLINE'S TOO CHEAP. WE DO NOT HAVE PRINTERS FOR ACARS MESSAGES. QUOTED XMISSIONS ABOVE ARE PARAPHRASES. I WAS TOO TIRED TO REMEMBER EXACT QUOTES. FLYING WITH A NEW (JUST OUT OF SOE) FO IS TOO FATIGUING FOR 6 LEGS. 99% OF THE TIME; NOTHING GOES WRONG ON THIS FLT IT DID. ATC MAY HAVE CALLED EHE NOT FHE. FLCS CAN'T REWIND THE TAPE TO FIND OUT. WHEN AN URGENT TONE OF VOICE IS USED TO TELL FHE TO DSND AND FHE DOES NOT HAVE ANY KNOWLEDGE OF EHE; FHE IS GOING TO DSND. FHE AND EHE MAY HAVE BLOCKED EACH OTHER'S XMISSIONS. BLOCKED XMISSIONS ARE COMMON AND DANGEROUS.