ACR Y UNAUTHORIZED DSCNT FROM ASSIGNED ALT BECAUSE OF SIMILAR NUMBERS.

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THIS RPT CONCERNS A FUNDAMENTAL BREAKDOWN OF A SYS OF CHKS AND BALS ALREADY IN EXISTANCE. THE FOLLOWING CONCERNS AN ALTDEV AND A MISSED READBACK BY ME. TYPICAL AFTERNOON SHIFT WORKING APCH CTL INTO DCA. ALTHOUGH I DIDN'T CONSIDER WORKLOAD TO BE ANYTHING LESS THAN MODERATE; REVIEW SHOWS HVY. MADE A XMISSION FOR ACR X TO DSND AND MAINTAIN 6000; HEAR ACR Y MAKE WHAT I THINK TO BE INITIAL CALL AFTER HDOF FROM IAD APCH. EXPERIENCE TOLD ME THAT QUITE POSSIBLY ACR Y MAY HAVE MISCONSTRUED THE DSCNT AS HIS. HAD TO SPECIFICALLY RESTATE THE DSCNT TO ACR X BEING VERY SPECIFIC TALKING TO ACR X. CASE OF TOO MANY THINGS TO DO AND NOT ENOUGH TIME TO DO IT ALL SO YOU PRIORITIZE IN YOUR MIND WHAT COMES FIRST. IN THIS CASE ACR X REQUIRED #1 PRIORITY DUE TO TFC. MY SCAN IS ON ACR Y DATA BLOCK; THE MODE C SHOWS ACR Y OBSERVED DSNDING BELOW ASSIGNED ALT (8000 IN THIS CASE). BEFORE I CAN CORRECT THIS SITUATION ACR Y KEYS THE MICROPHONE AND ASKS FOR VERIFICATION THAT DSCNT TO 6000 (ALT ASSIGNED ACR X) WAS FOR HIM. IMMEDIATELY I SAY NEGATIVE AND GIVE HIM 8000 BUT THE ACFT WAS SHOWN BY RADAR DUMP TO HAVE DSNDED TO 7400 FT; AT THE SCOPE I BELIEVE I OBSERVED ALT TO BE 7100 AT THE LOWEST. PLT TELLS ME THAT HE READ BACK 6000 TO WHICH SUBSEQUENT REVIEW OF TAPE SUBSTANTIATES LOUD AND CLR. I MISSED THE READBACK. ALL MY PLANES WERE ON THE SAME FREQ SO I CAN'T CONTRIBUTE IT TO CONGESTION ON SEVERAL FREQS. POS WERE COMBINED AND IT WAS WARRANTED. SEVERAL THINGS DISTURB ME ABOUT THIS. #1 IS THAT I MISSED A VERY CLR READBACK FROM THE WRONG AIRPLANE. MY FOCUS WAS SO INTENT ON DSNDING THAT FOR SOME REASON ALL ELSE GOT BLOCKED OUT IN MY MIND; BUT ONLY FOR A SPLIT SECOND. THE BUSINESS WE'RE IN MAKES THINGS HAPPEN FAST. SUPPLEMENTAL INFO FROM ACN 214036: BOTH CAPT AND I HEARD APCH CTL CLRNC FROM 8000 TO 6000 FT. CAPT READ BACK DSCNT CLRNC TO 6000 FT; BEGAN DSCNT WHEN ANOTHER ACFT WITH SIMILAR CALL SIGN CALLED APCH TO CONFIRM DSCNT CLRNC. SOME SCATTERED CUMULO NIMBUS IN AREA; SOME DEVIATING; AND CTL BECOMING RATHER LOADED UP. CAPT QUERIED OUR DSCNT CLRNC AND WE WERE TOLD TO MAINTAIN 8000 FT AND THAT HE WAS TOO BUSY TO RESPOND TO OUR 'ERRONEOUS READBACK.' STOPPED DSCNT AT 7500 FT AND CLBED BACK TO 8000 FT.