ZID CTLR EXPERIENCED OPERROR WITH AN ARR AND DEP AT 15000 FT.

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C500 DEPARTED LOUISVILLE AND WAS CLRED TO 14000 FT. CRJ WAS INBOUND TO CINCINNATI CLRED TO 15000 FT. C500 ASKED TO DEVIATE. I CLRED HIM TO DEVIATE L AND TO MAINT 14000 FT. CRJ ALSO ASKED TO DEVIATE. I CLRED HIM TO DEVIATE L OFF COURSE AND VERIFIED THE ASSIGNED ALT OF 15000 FT. WHEN I NOTICED THE C500 ABOVE IN 14000 FT I ASKED HIM TO VERIFY 14000 FT. HE SAID HE WAS CLBING TO 15000 FT. I TOLD HIM THAT I HAD ONLY CLRED HIM TO 14000 FT AND TO MAINTAIN 14000 FT. THE PLT RESPONDED BY SAYING 'WELL SOMEBODY CLRED ME TO 15000 FT.' I CALLED TFC TO THEM BOTH AND THE CRJ TOLD ME THAT HE HAD RECEIVED AN RA. THE DATA BLOCK SHOWED THE CRJ AT 15100 FT. AFTER HEARING THE TAPES; I VERY CLRLY ISSUED 14000 FT TO THE C500 BOTH ON INITIAL CALL AND WHEN I REITERATED AT THE TIME OF THE DEV REQUEST. HIS READBACK THE SECOND TIME WAS NOT CLR AND I WASN'T ABLE TO PREVENT THE SIT IN TIME. I THOUGHT I WAS HELPING BY RESTATING THE ALT WHEN IN FACT; IT HURT ME. THE PLT ON THE OTHER HAND; IS NOT HELD RESPONSIBLE FOR HIS ACTIONS OR HIS REMARKS ABOUT THE SIT.