SMT HAS NMAC WITH SECOND ACFT. SMT ON IFR FLT PLAN; SECOND ACFT APPARENTLY VFR.

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WHILE FLYING IFR AT 7000 FT UNDER THE CTL OF CLARKSBURG APCH; IN AN SMT; TRAVELING FROM CHARLESTON TO MORGANTOWN; I OVERHEARD AN ACFT ADVISED BY CLARKSBURG APCH OF MY POS IN FRONT OF AND BELOW HIM WITH ONLY A FEW MI OF SEPARATION. TO MY AMAZEMENT HE RPTED OUT OF 7500 FT DSNDING. CLARKSBURG RESPONDED TO MY IMMEDIATE EXPRESSION OF EXTREME CONCERN BY ADVISING THE ACFT TO RETURN TO 7500 FT. DESPITE VIGILANT SEARCHING I DID NOT SEE HIM; I WAS UNABLE TO TAKE EVASIVE ACTION. WITHIN SECONDS; IN A BLUR OF MOTION HE PASSED DIRECTLY OVERHEAD MY ACFT; WHICH HAD REMAINED LEVEL AT 7000 FT; AS ASSIGNED. THE 'SEPARATION' WAS LESS THAN I HAVE SEEN AS A SPECTATOR AT AIRSHOWS. IN OVER 20 YRS OF FLYING I HAVE NEVER FELT SO THREATENED OR VULNERABLE. I CAN FIND NO FAULT WITH THE AIR TFC CTLR WHO I SUSPECT WAS AS ALARMED AS I WAS AT THE PLT'S ACTIONS. I HAVE NO EXPLANATION WHY HE WOULD DSND TOWARDS TFC HE ACKNOWLEDGED WITHOUT HAVING ABSOLUTE VISUAL CONTACT. SIMILARLY IT IS BEYOND MY UNDERSTANDING WHY; IF HE DID SEE ME (AND I HAVE NO RECOLLECTION OF HIM ADVISING CLARKSBURG THAT HE DID IN FACT HAVE VISUAL CONTACT) HE DSNDED SO AS TO CROSS DIRECTLY OVERHEAD MY ACFT AT A FRIGHTENINGLY CLOSE DISTANCE.