DH8 CREW STARTED A DSCNT ON CLRNC MEANT FOR A COMPANY ACFT.

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AT APPROX XA30 LCL; CONTACT WITH ZSE WAS MADE AND THEY WERE SLOWING TFC DUE TO WX IN PDX. ANOTHER DHC8 WAS IN FRONT OF US AND THEIR COM WITH ZSE WAS DIFFICULT. THE PLT'S ENGLISH WAS POOR AND EACH COM WAS REPEATED AT LEAST TWICE. WE WERE GIVEN A 360 DEG TURN FOR SPACING; THEN ASKED TO KEEP IT TIGHT. THEN CLRED TO DSND TO 10000 FT. ALL CALLS WERE VERY CLOSE TO EACH OTHER; ALL REPEATED BY US WITH NO QUESTION OR VERIFICATION. AFTER DSNDING 600-800 FT; THE OTHER ACFT CALLED AND ASKED IF THE DSCNT WAS FOR THEM. THAT IS WHERE IT GOT CONFUSING. WE THOUGHT THE CLRNC WAS FOR US AND NO ONE QUESTIONED OUR REPLY. MY BEST GUESS IS; AS USUAL; COMS BECAME STRESSED; AND WE MADE A MISTAKE; DUE TO BAD CALLS THE CTLR WAS EXTREMELY BUSY; AND THE OTHER ACFT WAS NOT STAYING WITH ANY OF THEIR CALLS; CAUSING FRUSTRATION FOR ALL. NOT KNOWING WHAT WAS HAPPENING IN THE OTHER ACFT; WE REQUESTED NEW ALT CLRNC AND ZSE CLRED US TO 10000 FT. NO CONFLICT EXISTED OR ANY EVASIVE ACTION TAKEN. THE ONLY CONTRIBUTING FACTOR OTHER THAN COMS; WOULD BE FATIGUE. ON A 5 WK SCHEDULE ALL OTHER TRIPS SHOW AROUND XA00 AND FINISH BTWN XK00-XM00. EXCEPT THIS TRIP; ONE ODDBALL WITH AN XQ00 RPT THAT DAY AND 6 LEGS. OUR INTERNAL CLOCKS WERE DEFINITELY NOT THEIR NORM. OUR COMPANY IS TRYING TO HAVE ALL TRIPS OF THE SAME TYPE; AM/PM.