MLG; DSNDING TO 1000 FT ABOVE A DASH 8; GOT A FALSE GPWS WARNING WHEN THE EQUIP PERCEIVED THE OTHER ACFT TO BE THE TERRAIN. COMPLYING WITH THEIR COMPANY'S FOM; THEY CLBED FROM 11000 FT ASSIGNED ALT TO ALMOST 12000 FT.

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LINE CHK BTWN DENVER AND SEATTLE. CLRED FOR CHINS 2 ARR (CHINS.CHINS2) SEATTLE; WA. IN VICINITY OF CHINS INTXN; SEA APCH ADVISED US OF DASH 8 TFC BELOW US; FLYING THE SAME ARR. SHORTLY THEREAFTER; WE WERE GIVEN CLRNC TO DSND TO 11000 FT MSL. UPON REACHING 11000 FT; THE DASH 8 WAS 1000 FT BELOW US AS INDICATED ON TCASII. AS WE OVERTOOK THE DASH 8; WE GOT A GPWS 'TERRAIN; TERRAIN' ANNOUNCEMENT. OUR COMPANY FLT MANUAL CALLS FOR THE FOLLOWING RESPONSE IN NIGHT CONDITIONS: IMMEDIATE ACTION: THRUST -- MAX. PITCH ATTITUDE -- ROTATE TO 15 DEGS (INCREASE PITCH ATTITUDE UNTIL WARNING STOPS). ATC -- ADVISE. WARNING: DO NOT DELAY PULL-UP FOR DIAGNOSIS. BECAUSE OF OUR TRAINING; THE CAPT INITIATED THE IMMEDIATE ACTION ITEMS. PRIOR TO REACHING 12000 FT MSL (1000 FT ABOVE ASSIGNED ALT) THE AURAL WARNING STOPPED. WE ADVISED ATC OF OUR ALTDEV AND WERE GIVEN CLRNC AND ACKNOWLEDGEMENT TO RETURN TO 11000 FT MSL. THE GPWS REACTED TO AN ACFT TARGET WITH 1000 FT SEPARATION AS IF IT WERE TERRAIN. WE WERE AWARE OF THE TFC PRIOR TO THE 'TERRAIN; TERRAIN' WARNING; BUT RESPONDED IN ACCORDANCE WITH THE IMMEDIATE ACTIONS FOR AN EMER PROC AS WRITTEN IN OUR COMPANY FLT MANUAL.