ACR SF34 FLC APPARENTLY ACKNOWLEDGED DSCNT CLRNC FOR ANOTHER ACFT; AND COMES INTO CONFLICT WITH POSSIBLY ANOTHER ACFT. FLC BECOMES CONFUSED WHEN ACKNOWLEDGING MULTIPLE FREQ CHANGES AND DOES NOT ATTEMPT CONFIRMATION WHEN CLRNCS 'SEEM SUSPECT.'

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ENRTE EBOUND FROM ISP TO BOS; WE WERE AT 8000 FT MSL ON NEW YORK APCH FREQ 118.00. WE WERE APPROX 1/2 WAY BTWN HTO AND GON VORS. WE WERE THEN HANDED TO NEW YORK APCH FREQ 132.25. SOON WE WERE THEN TOLD TO MAINTAIN 10000 FT MSL AND HANDED BACK TO NEW YORK APCH FREQ 118.0. I ACKNOWLEDGED '10000 FT AND 118.00.' WE CLBED TO 10000 FT MSL AND ATTEMPTED TO CONTACT NEW YORK ON 118.00. SEVERAL ATTEMPTS WERE MADE TO 'CHK IN' BEFORE RECEIVING A RESPONSE FROM ATC. THE CTLR SAID THAT WE WERE SUPPOSED TO BE ON 132.25. I INFORMED THEM THAT WE HAD JUST COME FROM 132.25 BUT THAT WE WOULD GO BACK TO THEM. AT SOME TIME IN THE HDOF CONFUSION AND SWITCHING FREQS; WE NOTED OPPOSITE DIRECTION TFC 2.5 MI AWAY ON TCASII AT 10000 FT MSL AND TURNED APPROX 20 DEGS L OF COURSE TO ENSURE SEPARATION. I BELIEVE THE TFC SHOWED AS A 'TA' ON TCASII. THE CAPT AND MYSELF COMMENTED THAT WE WERE SURPRISED THAT ATC HAD NOT POINTED OUT THE CONFLICTING TFC. THIS EVENT OCCURRED BEFORE WE HAD A CHANCE TO RETURN TO 132.25. UPON RETURNING TO 132.25; THE CTLR IMMEDIATELY ASKED US OUR ALT. I STATED 10000 FT. WITH SOME URGENCY IN HIS VOICE HE TOLD US TO RETURN TO 9000 FT; PROBABLY DUE TO THE OPPOSITE DIRECTION TFC WHICH BY THIS TIME WAS NOT A FACTOR. HE THEN ASKED WHO GAVE US 10000 FT AND THE FREQ CHANGE. I SAID THE PREVIOUS CTLR. BY THIS POINT; SO MUCH CONFUSION EXISTED THAT THE CAPT AND I COULD NOT BE SURE WHICH FREQ WE WERE ACTUALLY ON WHEN WE WERE TOLD TO MAINTAIN 10000 FT. ALTHOUGH WE DID NOT REALIZE IT AT THE TIME; THERE WERE SOME ITEMS THAT DID NOT SEEM RIGHT. FIRST; THE FLT PLANNED RTE WAS FOR 9000 FT MSL AND WE WERE NOW AT 10000 FT MSL. SECOND; WE HAD JUST COME FROM 118.00 TO 132.25 SO WHY WOULD WE BE HANDED BACK TO 118.00. IN THAT MOMENT; WE WERE SIMPLY COMPLYING WITH ATC'S INSTRUCTIONS. MY CAPT AND I DISCUSSED THE EVENTS THAT HAD JUST OCCURRED AND WONDERED HOW WE COULD HAVE GOTTEN TO 10000 FT IF ATC ACTUALLY WANTED US AT 9000 FT. I HAD READ BACK THE ALT ASSIGNMENT (ALONG WITH THE FREQ CHANGE) AND SELECTED IT IN THE ALT SELECTOR. AFTER REVIEWING THE SIT; MY ONLY GUESS IS THAT MAYBE I ACKNOWLEDGED AN ALT AND FREQ CHANGE THAT WAS ACTUALLY GIVEN TO ANOTHER ACFT GOING THE OPPOSITE DIRECTION. THIS MAY EXPLAIN WHY WE WERE GETTING HANDED BACK TO 118.00 AND GIVEN 10000 FT; ALL POSSIBLE INSTRUCTION FOR A WBOUND ACFT. NO ACKNOWLEDGEMENT FROM ATC THAT MY READBACK WAS INCORRECT OR THAT THE COMMAND WAS ACTUALLY FOR ANOTHER ACFT WAS RECEIVED; AS FAR AS I KNOW. IT'S POSSIBLE THAT THE CTLR WAS NOT VIGILANT IN LISTENING TO MY READBACK OR THAT I SWITCHED FREQS BEFORE THE CTLR HAD THE OPPORTUNITY TO CORRECT ME. AS FOR THE REASON THAT ATC DID NOT POINT OUT TFC TO US MAY HAVE BEEN THAT WE WERE 'BTWN' FREQS AS A RESULT OF THE HDOF CONFUSION THAT WAS TAKING PLACE. THIS IS A GOOD EXAMPLE OF THE TCASII SYS DOING ITS INTENDED JOB EVEN THOUGH UNDER POSITIVE ATC IFR CTL. IN THE FUTURE I PLAN TO TREAT ALL ALTS THAT ARE DIFFERENT THAN THAT IN MY CLRNC/FLT PLAN WITH SUSPECT.