A ZID Center Controller reported a loss of separation between two aircraft when one of the aircraft cleared direct KENLL mistakenly programmed the fix KENLN into their navigation.

Narrative

Aircraft Y was climbing to 23;000 ft. off LUK and was traffic with Aircraft X climbing to 23;000 ft. off LOU. I had called CVG ATCT several times to try to point out Aircraft X on an eastbound heading. The supervisor realized I was spending a lot of time attempting to reach the Approach sector for coordination so he gave me a D-side. When I finally did get in contact with CVG ATCT they approved my point out and asked what I wanted them to do with his potential traffic Aircraft Y climbing southbound. I told them they could leave Aircraft Y on course but 'my control'. So I turned Aircraft X 15 degrees left. I also turned Aircraft Y 15L on contact. Around FLM; the two aircraft were laterally separated so I pointed out Aircraft Y to sector 26 and got control to turn him on course. 26 released control for turns so I cleared Aircraft Y direct KENLL; which was the first fix after HYK and I pronounced it 'Kennell'. The pilot of Aircraft Y also read back 'Kennell' but made roughly a 50 degree right turn to a fix on his departure KENLN; often pronounced 'Keenlin'; which turned him back into conflict with Aircraft X. When I realized the Aircraft Y pilot had made an error in his turn I turned Aircraft Y 20 degrees left and stopped Aircraft X's climb at 18000 ft. There was a loss of separation of 4.59 miles and 800 ft. I switched the Aircraft Y to the high altitude stratum once the conflict was resolved and the 83/93 radar controller read the Aircraft Y the brasher statement for a pilot deviation. Aircraft departing CVG and LUK should not have both KENLN and KENLL in their route. I should have clarified direct KENLL; not KENLN; but having both of those fixes in the same route is the main reason for the confusion and for the loss of separation.