SIMILAR CALL SIGNS; FREQ CONGESTION. FLC RESPONDED AND INITIATED OTHER ACFT'S CLRNC.

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FLT ABCD; COS-DEN; 18 MINS ENRTE; ALT 12000 FT; HDG 320 DEGS. IT WAS THE FO'S LEG. THE CAPT HAD JUST GOTTEN THE ATIS AND WAS COMING BACK ON COM #1 FOR ATC AS THE FO TOOK A HDOF TO DEN APCH. APCH CTL WAS AN EXTREMELY BUSY FREQ AT THE TIME WITH IFR CONDITIONS; MODERATE TURB; AND SNOW. A CALL CAME FOR FLT ABCE TO DSND TO 11000 FT. AFTER NO RESPONSE AND SOME HESITATION; THE CAPT ANSWERED 'UNDERSTAND FLT ABCD CLRED TO 11000 FT.' WITH NO RESPONSE TO THIS CALL EXCEPT THE CONTINUOUS CHATTER BTWN APCH CTL AND OTHER ACFT; WE ACCEPTED THIS CLRNC AS BEING MEANT FOR US; AND STARTED TO DSND. WITHIN A FEW SECONDS; ANOTHER CLRNC CAME FOR FLT ABCE TO TURN L TO 290 DEGS. AGAIN; NO RESPONSE. WE HESITATED; THEN THE CAPT AGAIN QUERIED THE CTLR THAT THE CLRNC WAS FOR FLT ABCD; AS WE BEGAN TO SUSPECT SOMETHING WAS WRONG AND LEVELED OFF AT 11500 FT. WE WERE EVENTUALLY INFORMED THAT THE CLRNC WAS FOR FLT ABCE. WE MAINTAINED HDG OF 320 DEGS; DECIDED THAT WE HAD NOT BEEN CLRED TO 11000 FT; AND PROCEEDED TO CLB BACK TO 12000 FT. THE FLT CONTINUED TO DEN WITHOUT FURTHER INCIDENT.