C172 FOLLOWS WRONG HELI FOR LNDG AT HEF.

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I WAS FLYING A CESSNA 172N LNDG AT MANASSAS REGIONAL ARPT IN VIRGINIA. THE PATTERN WAS VERY BUSY WITH APPROX 5-7 LNDG ACFT BEING HANDLED AT ONCE. I WAS TOLD TO OVERFLY THE ARPT AT 2000 FT AND ENTER R BASE FOR RWY 34L; WHICH I DID AND DSNDED TO TPA (1200 FT). THE CTLR TOLD ME THAT I WAS 'CLRED TO LAND #3 AFTER THE ROBINSON HELI.' I REPEATED THIS AND TOLD HIM THAT I WAS TURNING BASE THIS TIME. HE RESPONDED THAT I MUST NOT TURN BASE TILL I HAD THE TFC IN SIGHT. I CONTINUED ON THE DOWNWIND UNTIL I HAD A HELI IN SIGHT AND RPTED IT IN SIGHT. I TURNED BASE; FINAL AND FOLLOWED THAT HELI IN. DURING THE FINAL APCH I SAW ANOTHER HELI THAT WAS HOVERING AT AN ALT I ESTIMATED TO BE APPROX 1000 FT OVER THE ARPT. IT WAS NOT MOVING EITHER VERTLY OR HORIZLY. I CONTINUED THE APCH; KEEPING THE HELI IN SIGHT; LANDED AND TAXIED OFF THE RWY NEAR WHERE THE HELI I FOLLOWED HAD EXITED. THE CTLR TOLD ME TO CALL THE TWR ON SHUTDOWN AND GAVE ME A PHONE NUMBER. I MADE A NOTE OF THE TAIL NUMBER OF THE HELI THAT I FOLLOWED OFF. I CALLED THE TWR; HE ASKED ME IF I KNEW WHAT I HAD DONE. I RESPONDED THAT I DID NOT; AND HE INFORMED ME THAT I HAD LANDED '...UNDERNEATH ANOTHER ACFT...' THE HELI. I RESPONDED THAT I FOLLOWED THE HELI (REPEATED THE TAIL NUMBER THAT I HAD NOTED) IN AND IT WAS CLR OF THE RWY WHEN I LANDED. HE SAID THAT I SHOULD HAVE BEEN AWARE THAT THERE WAS MORE THAN 1 HELI IN THE PATTERN AND THAT WAS NOT THE ONE I WAS TO FOLLOW. THE ONE HOVERING WAS THE ONE I WAS TO LAND AFTER. AT NO TIME DURING THE FINAL APCH WAS ANY ADVISORY TO ME (TO SLOW DOWN OR GO AROUND) OR THE HOVERING HELI GIVEN. I APOLOGIZED TO HIM; AND THEN DROVE TO WHERE THE HELI WAS AND APOLOGIZED TO THEM FOR LNDG UNDERNEATH THE HOVERING HELI AND EXPLAINED THE SIT. THE FBO WAS VERY GRACIOUS ABOUT IT AND INDICATED THAT THERE WAS NO PROB AND APPRECIATED MY EXPLANATION. I HAD THE HOVERING HELI IN SIGHT DURING THE ENTIRE APCH AND LNDG; AND SEEING NO VERT OR HORIZ MOVEMENT THOUGHT THAT HE WAS HOVERING THERE AND NOT LNDG. ALSO; SINCE I SAW THE OTHER HELI CLR THE RWY; I THOUGHT I WAS NEXT TO LAND. THERE WAS NO IMMINENT DANGER OF COLLISION; JUST A MIX-UP IN COM. I AM NOT TRYING TO AVOID RESPONSIBILITY FOR SEE AND AVOID.