TYS controller described a confused routing event when; absent any information; agreed to file a flight plan but filed the incorrect destination airport; not clarifying the airport name and state.

Narrative

I was working the FD/CD (Flight Data/Clearance Delivery) position at Knoxville when an Aircraft X called looking for an IFR flight plan that he claimed he had filed to Jacksonville. After checking my strips and the FDIO I told him that I didn't show a flight plan on file. I told him that he could contact FSS to file his flight plan or since I wasn't busy; I could file it for him. He said it would be great if I would file it for him; so I asked him for his planned route. He said he wanted to go to Jacksonville via McGee Tyson direct SPA; which he spelled out phonetically; Sierra Papa Alpha; and then direct Jacksonville. I then asked him for his airspeed and requested altitude and after receiving these items I input his flight plan. In the route portion of the flight plan I input; TYS … SPA … JAX. After completion of inputting his fight plan information and receiving a flight progress strip; I read him his clearance; including clearing him to the JAX airport as filed. He read it back correctly and I passed his strip on to the Local Controller. Later on that day I was observing the operation in the TRACON of our facility when one of our FML's came in and asked the Radar Controller if he had the strip on the DC9. After going through the filed flight progress strips; he found it. As he was leaving; I said; 'I filed that guy; was there a problem?' He informed me that he had been filed to the incorrect destination; and that he wanted to go to Ellis Field in Jacksonville; NC (OAJ) not to Jacksonville; FL (JAX) as I had filed him. This airport is a small uncontrolled airport in rural NC. I haven't been briefed on what exactly happened in the center sector when this aircraft navigated in a different direction than the center sector expected. I have been informed that a loss of separation fortunately did not occur. I; as the FDCD (Flight Data/Clearance Delivery) position; have the responsibility to confirm the exact routing of a flight plan I will be filing. I will be sure to do so in similar cases in the future. I also believe that if a pilot wants to file a flight plan to an airport that could be easily misconstrued for one of the same name that's in well known major metropolitan that he should be more specific to avoid an easily made assumption from turning into a major problem.