ARTCC CTLR AT ZID ISSUED A DSCNT CLRNC AND A FREQ CHANGE TO ACR X TO FL290. ACR Y WITH A SIMILAR CALL SIGN RESPONDED SIMULTANEOUSLY AND ALSO SWITCHED FREQ.

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IN THIS SIT; ACR X WAS ISSUED A DSCNT CLRNC TO FL290 AND A FREQ CHANGE TO 124.57. BOTH ACR X AND ACR Y READ BACK THE CLRNC SIMULTANEOUSLY AND THEN SWITCHED TO THE STATED FREQ. THE DSCNT OF ACR Y WHOM THE CLRNC WAS NEVER INTENDED; PUT THE ACFT IN IMMEDIATE PERIL WITH AN OVERFLT AT FL290; ACFT Z. THE ISSUANCE OF AN ALT AND A FREQ CHANGE IN THE SAME CLRNC RESULTED IN THE WORST CASE SCENARIO. 2 ACFT READ BACK THE ALT AND NOT KNOWING THAT ANOTHER ACFT WAS ALSO READING BACK THE SAME CLRNC; IMMEDIATELY SWITCHED TO THE ASSIGNED FREQ. EVEN THOUGH THE CTLR FAILED TO GET A POSITIVE READBACK; HIS IMMEDIATE EFFORTS TO DO SO WERE IN VAIN AS BOTH ACFT HAD SWITCHED. THE ISSUANCE OF AN ALT AND A FREQ CHANGE IN THE SAME CLRNC SHOULD BE PROHIBITED. IT HAS ALWAYS BEEN CONSIDERED 'BAD FORM' ANYWAY. SUPPLEMENTAL INFO FROM ACN 439894: FACTS: WE WERE FLYING AT OUR ASSIGNED ALT OF FL310. WE HEARD A CLRNC FOR OUR FLT TO DSND TO FL290. WE LEVELED OFF AT FL290. AFTER SEVERAL MINS; THE CTLR ADVISED US TO CALL ZID AT OUR NEXT STOP ABOUT A POSSIBLE PLTDEV. AT FL290; WE NEVER SAW ANY CONFLICTING TFC; WE NEVER RECEIVED A TCASII WARNING (TA OR RA). WE WERE NEVER AWARE OF ANY TYPE OF CONFLICT. A SUBSEQUENT PHONE CALL WITH ZID QUALITY ASSURANCE OFFICE INDICATED THAT ANOTHER FLT WITH A SIMILAR FLT NUMBER HAD BEEN GIVEN THE DSCNT TO FL290; AND A FREQ CHANGE IN THE SAME XMISSION. THE CTLR RECEIVED A GARBLED RESPONSE (IMPLYING THAT 2 ACFT RESPONDED SIMULTANEOUSLY). WHEN THE CTLR TRIED TO CLARIFY AND ENSURE THAT ONLY 1 ACFT HAD TAKEN THE CLRNC; THERE WAS NO RESPONSE -- BOTH ACFT HAD ALREADY CHANGED FREQS. SUBSEQUENTLY; A SEPARATION CONFLICT OCCURRED BTWN MY FLT AND ANOTHER ACFT AT FL290. RECOMMENDATIONS: 1) ATC SHOULD ALWAYS ADVISE PLTS WHEN ACFT WITH SIMILAR CALL SIGNS ARE ON THE SAME FREQ. WE WERE NOT ADVISED OF THE SIMILAR CALL SIGNS. 2) ATC SHOULD AVOID ASSIGNING AN ALT CHANGE AND FREQ CHANGE IN THE SAME XMISSION. IF WE HAD BEEN GIVEN THE ALT CHANGE FIRST; IN A SEPARATE XMISSION FROM THE FREQ CHANGE; WE WOULD HAVE STILL BEEN IN CONTACT WITH THE CTLR WHEN HE TRIED TO VERIFY THE CLRNC. 3) PLTS SHOULD BE ENCOURAGED TO MOMENTARILY DELAY A FREQ CHANGE WHENEVER THEY RECEIVE A FREQ CHANGE AND ALT CHANGE IN THE SAME XMISSION. HAD WE DELAYED 'FLIPPING THE SWITCH' TO THE NEW FREQ; WE PROBABLY WOULD HAVE HEARD THE CTLR'S ATTEMPT TO VERIFY THE CLRNC.