C177 AND PA28 HAVE NMAC WHILE UNDER THE RADAR CONTROL OF A D21 CONTROLLER.

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I WAS ENRTE FROM ZZZ1 DIRECTLY TO ZZZ2 ON AN IFR FLT PLAN IN A CARDINAL 177. MY ACFT HAD AN ACTIVE XPONDER WITH MODE C CAPABILITIES AND VMC APPLIED TO THE ENTIRE PORTION OF MY FLT. I HAD BEEN HANDED OFF TO DETROIT APCH FROM FLINT APCH AROUND XA30; AT WHICH TIME I WAS ON A HDG OF 150 DEGS; AND IN LEVEL FLT AT 5000 FT MSL. A CTLR AT DETROIT APCH THEN INSTRUCTED ME TO DSND AND MAINTAIN 4000 FT MSL AND PROCEED DIRECT TO ZZZ2 AROUND XA35. I COMPLIED AND DSNDED TO 4000 FT MSL; THEN MAINTAINED LEVEL FLT AT THAT ALT. NO HDG CHANGE WAS REQUIRED SINCE MY CURRENT HDG OF 150 DEGS ALREADY HAD ME GOING DIRECT TO ZZZ2. APPROX 5 MINS AFTER LEVELING OFF AT 4000 FT; WHAT LOOKED LIKE A PIPER ARROW DSNDED DOWN AND IN FRONT OF ME AT A DISTANCE OF APPROX 100 FT. THE PIPER WAS TRAVELING APPROX 15 KTS FASTER THAN MY ACFT AND APPEARED TO BE COLORED BLUE ON WHITE. NEITHER THE PLT OF THE PIPER NOR I COULD HAVE SEEN EACH OTHER SINCE HE HAD A LOW WING PLANE AND WAS ABOVE ME; AND I HAD A HIGH WING ACFT AND WAS BELOW HIM. I'M FAIRLY SURE THAT THE PLT OF THE PIPER NEVER SAW ME OR HAD ANY IDEA OF HOW CLOSE WE CAME TO COLLIDING. AS SOON AS THE PIPER ACFT GOT IN FRONT OF ME; I PUT MY ACFT TO A NOSE-DOWN ATTITUDE AND BANKED R TO A HDG OF 180 DEGS TO OFFSET MY COURSE FROM THAT OF THE OTHER ACFT. I QUESTIONED THE CTLR AS TO WHY SHE DSNDED ANOTHER ACFT DIRECTLY INTO MY FLT PATH. THE CTLR RPTED BACK TO ME THAT SHE WAS VECTORING THE PIPER ACFT INTO ZZZ2 AS WELL; AND HAD THE PIPER DSNDING DOWN TO 3000 FT MSL. THE ATC CTLR SAID THAT SHE HAD TAGGED US AS THE SAME PLANE SINCE OUR XPONDER AND RADAR PICKUPS WERE SO CLOSE TO EACH OTHER. THE CTLR DID NOT SEEM CONCERNED OR BOTHERED THAT SHE HAD NEARLY CAUSED A MIDAIR COLLISION BTWN OUR PLANES; AND DIDN'T HAVE ANY GOOD ANSWERS FOR ME AS TO HOW A MISTAKE LIKE THAT COULD HAPPEN. I THEN PROCEEDED DIRECT TO ZZZ2 STILL AT 4000 FT. ABOUT 15 MI OUT FROM ZZZ2; I TOLD THE CTLR THAT I HAD THE ARPT IN SIGHT AND WOULD DSND TO 3000 FT AND CANCEL IFR.