ACR X DSCNT TO OCCUPIED ALT POSSIBLE LTSS FROM ACR Y. POSSIBLE SYS ERROR.

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ACR X APCHING IAD FROM THE S WE WERE UNABLE TO ACCEPT THE USUAL COATT 3 ARR DUE TO TSTMS. CB'S ALONG THE ARR RTE. INSTEAD WE DEVIATED TO THE R AND WERE GIVEN ALTERNATE ROUTING VIA PXT; V31 TO OTT TO IAD. IN THE VICINITY OF OTT WE WERE TOLD TO DSND FROM FL190 TO 4000 FT. WHEN APPROX 4 MI W OF DCA DSNDING THROUGH APPROX 12000 FT (IN IMC CONDITIONS) ATC (WASHINGTON APCH) TOLD US IN A VERY EXCITED TONE TO TURN R TO A HDG OF 360 DEG AND TO LEVEL AT 9000 FT. AFTER THE TURN WAS COMPLETED WE WERE STILL DSNDING AND WE WERE TOLD TO TURN TO A 250 DEG HDG. TURN WAS STARTED WHEN IN A VERY EXCITED TONE WE WERE INSTRUCTED TO MAINTAIN OUR PRESENT ALT (APPROX 10300 FT). WE LEVELED OFF AND AT THAT POINT WE COULD SEE ACR Y APPROX 1000 FT BELOW US AND AT THE 10:30 POS AT 1 1/2 MI HEADED IN A NORTHERLY DIRECTION. WE RPTED ACR Y IN SIGHT AND WERE SUBSEQUENTLY CLRED WITHOUT FURTHER INCIDENT. THE CEILING AT THAT POINT WAS APPROX 10500 FT AND RAGGED. AT ALL TIMES WE COMPLIED WITH ATC INSTRUCTIONS. IT APPEARED THAT ATC HAD MOMENTARILY LOST TRACK OF EITHER OUR ACFT OR ACR Y TFC BELOW US TO THE POINT WHERE WE HAD LESS THAN MIN SEP.